Lunar lava tubes are vast underground cavities left behind by ancient volcanic flows on the Moon, and they may be among the most valuable pieces of real estate off Earth. Modeling studies suggest some could be more than a kilometer wide while remaining structurally sound, dwarfing anything comparable on our own planet. Because the Moon lacks the weathering forces that collapse most terrestrial caves over geological time, these tubes have likely persisted for billions of years, offering ready-made shelters from radiation, temperature swings, and micrometeorite bombardment that make the lunar surface so hostile.
How Lunar Lava Tubes Formed
Lava tubes on Earth form when the outer surface of a flowing lava stream cools and hardens into a crust while molten rock continues to move underneath. Once the eruption ends and the remaining lava drains away, it leaves a hollow tunnel behind. The same basic process operated on the Moon billions of years ago, during the period of intense volcanism that filled the large dark plains visible from Earth, the lunar maria. A comparative review of tubes on Earth, the Moon, and Mars found that deep inflation and thermal entrenchment was the dominant formation mechanism on the Moon, distinguishing it from the inflation-and-overcrusting process more common on Mars.1Icarus. The structural stability of lunar lava tubes
What makes lunar tubes so different from their terrestrial counterparts is scale. On Earth, lava tubes rarely exceed about 30 meters in width. On the Moon, the combination of lower gravity (roughly one-sixth of Earth’s) and the absence of tectonic activity and erosion allows tubes to grow far larger and persist far longer. A comparative planetology review found that Martian and lunar tubes are one to three orders of magnitude more voluminous than those on Earth.2ScienceDirect. Lava tubes on Earth, Moon and Mars: A review on their size and morphology revealed by comparative planetology Even at those enormous dimensions, modelers have found they remain within the threshold of roof stability.
How Large Can They Get
The question of just how wide a lunar lava tube can be before its roof collapses under its own weight has been studied through structural modeling. A widely cited analysis showed that with a roof only about 2 meters thick, a tube more than a kilometer wide can remain stable under lunar gravity. At greater burial depths of around 500 meters, and assuming favorable stress conditions in the surrounding rock, tubes up to 5 kilometers wide could theoretically hold together.3Icarus. The structural stability of lunar lava tubes To put that in perspective, a 5-kilometer-wide tube could comfortably contain a mid-size city’s downtown district.
These estimates are supported by gravity data from NASA’s GRAIL mission, which measured subtle variations in the Moon’s gravitational field from orbit. Mass deficits consistent with large underground voids have been detected beneath the lunar surface, and the structural modeling results align with what GRAIL data imply about tube size.4Icarus. The structural stability of lunar lava tubes The stability depends on a few key factors: the width of the tube, the thickness of the roof, and the stress state of the surrounding basaltic rock. Thicker roofs and deeper burial both work in favor of stability.
Finding Tubes You Cannot See
Since lava tubes are underground, detecting them from the surface or from orbit requires indirect methods. The most productive approach so far has used gravity data. The GRAIL spacecraft orbited the Moon in tandem pairs, measuring gravitational pull with extraordinary precision. Researchers developed gradiometry and cross-correlation techniques to pick out the signature of long, narrow, winding mass deficits buried beneath the surface. This work turned up several strong candidate tubes, some extending known surface channels called rilles, some sitting beneath recently discovered skylight openings, and some hiding under featureless terrain.5Geophysical Research Letters. Evidence of large empty lava tubes on the Moon using GRAIL gravity
A more recent analysis refined these methods for the Marius Hills region, a volcanic complex on the Moon’s near side that has long been considered a prime lava-tube hunting ground. Using a gravitational gradient method capable of detecting anomalies in both north-south and east-west directions, researchers identified a negative density anomaly near the Marius Hills Hole skylight, at about 57.5°W and 14.3°N. That signal, they concluded, indicates a potential lava tube, a finding corroborated by forward modeling and lunar radar echo data.6Icarus. GRAIL gravity gradients evidence for a potential lava tube at Marius Hills on the moon
Gravity is not the only tool in the kit. Ground-penetrating radar has been tested extensively in terrestrial lava tubes as preparation for future lunar missions. At Lava Beds National Monument in California, radar clearly resolved the ceilings of tubes with only a few meters of rock overhead, accurately estimating cave width when compared against lidar measurements taken inside the caves. The technique works less well when three-dimensional geometry becomes complex, but forward modeling using lunar parameters suggests it should perform similarly well on the Moon.7Journal of Geophysical Research: Planets. Resolution of Lava Tubes With Ground Penetrating Radar: The TubeX Project Magnetometry offers yet another approach. Researchers have mapped the magnetic signatures of terrestrial lava tubes and scaled their models to the lunar magnetic environment, demonstrating that magnetometer surveys from a rover could help locate and trace tubes on the Moon.8Journal of Geophysical Research: Planets. Field Mapping and Modeling of Terrestrial Lava Tube Magnetic Anomalies as an Analog for Lunar Lava Tube Exploration and Prospecting
Skylights as Windows Into the Underground
The most direct evidence that lava tubes exist on the Moon comes from skylights: places where a section of a tube’s roof has collapsed, creating a pit visible from orbit. Several of these have been photographed at high resolution by NASA’s Lunar Reconnaissance Orbiter. The Mare Tranquillitatis Pit and the Marius Hills Hole are the two most studied. Photogrammetric modeling using multiple stereo images from the LRO’s Narrow Angle Camera has produced detailed three-dimensional models of these pits at a spatial resolution of about 2 meters, allowing researchers to analyze their depth, diameter, and slope characteristics.9Earth and Space Science. High‐Resolution Morphology of Lunar Lava Tube Pits Using Photogrammetric Modeling of Multiple Stereo Images
These skylights are more than just interesting geology. They are potential entry points for future exploration. A collapsed pit that exposes the interior of an intact tube could serve as a natural doorway, though getting equipment and people safely down a vertical drop and back up again presents engineering challenges that are already being worked on.
Natural Shelter From a Hostile Surface
The lunar surface is a punishing environment. Without an atmosphere or a global magnetic field, it is fully exposed to cosmic radiation, solar particle events, and a constant drizzle of micrometeorites. Temperatures swing wildly depending on whether a given spot is in sunlight or shadow, ranging from about −170 °C during the lunar night to around 110 °C in direct sunlight. A lava tube’s rock ceiling changes all of that.
Inside a tube, the overlying rock blocks cosmic rays and energetic solar particles, eliminates the micrometeorite hazard, and buffers temperature extremes. Thermal modeling based on data from NASA’s Diviner radiometer experiment suggests that the interior of lava tubes near the Mare Tranquillitatis maintains a stable temperature of roughly 17 °C year-round. Even shadowed cave openings that are not fully enclosed stay within a range of about −20 to 30 °C, far milder than the surface.10International Journal of Mining Science and Technology. A comprehensive review of lunar lava tube base construction and field research on a potential Earth test site That 17 °C figure is close to a comfortable room temperature, and the stability matters as much as the absolute number: without dramatic thermal cycling, materials and structures inside a tube would face far less mechanical stress.
A comprehensive review described the internal environment of a lunar lava tube as a “benign alternative to the harsh lunar surface,” providing natural protection from radiation, micrometeoroids, and extreme temperature fluctuations.11Acta Astronautica. Pressurized lunar lava tubes for habitation Separate work on meteoroid impacts confirmed that intact tubes provide effective natural shielding from all three hazards.12Acta Astronautica. Investigation of meteoroid impacts on lunar lava tubes For any permanent or semi-permanent human presence, this combination of protections removes some of the most expensive and technically challenging requirements: you would not need to engineer massive radiation shielding or worry as much about micrometeorite impacts on habitat walls.
From Natural Cave to Pressurized Habitat
Using a lava tube as a shelter is not the same as living in one. The tube itself is in hard vacuum, so any human habitat would still need to be pressurized and sealed. The question is how to turn a raw cave into a functional living space. Several architectural and engineering concepts have been proposed.
One approach treats the tube as a ready-made protective shell and constructs inflatable or rigid pressurized modules inside it, much like assembling a space station within the cave. The tube’s rock walls absorb radiation and micrometeorites, while the engineered modules handle atmosphere, life support, and structural containment. Research into pressurized lunar lava tube habitats has explored how to seal sections of a tube and maintain internal pressure, taking advantage of the tube’s geometry to reduce the structural load on the habitat itself.13Acta Astronautica. Pressurized lunar lava tubes for habitation
Another line of work sidesteps the challenge of finding and accessing a natural tube entirely, proposing instead to build tube-like protective structures on the surface using local lunar soil (regolith) processed into construction material. One such concept uses parametric design to optimize large-span arch habitats that mimic a lava tube’s protective geometry.14Space Habitation. Parametric design and optimization for lava tubes-like lunar arch habitation with large span These “lava tube-like” structures borrow the best feature of the natural version, the overhead shielding, while being buildable anywhere on the lunar surface.
Life support inside any enclosed underground habitat brings its own complications. Because the space is sealed, contamination from equipment, experiments, or industrial processes cannot just vent away. Research into extraterrestrial lava tube habitation has flagged the need for stringent ventilation and environmental pollution monitoring within each module, with dedicated detection devices to keep the air clean.15ScienceDirect. Research on the impact of extraterrestrial lava tube environments on human survival and countermeasures In a sealed tube habitat, a contamination event that would be trivial on Earth’s surface could be genuinely dangerous.
Sending Robots In First
Before anyone walks into a lunar lava tube, robots will go first. Exploring a tube through a skylight means descending a vertical shaft into total darkness, navigating uneven terrain covered in rubble and dust, and doing it all without GPS or reliable communication with the surface. These challenges have driven development of cooperative robotic systems designed for exactly this kind of mission.
A mission concept validated on Lanzarote island in Spain demonstrated how a team of robots could work together to explore a lava cave. Surface robots mapped the area around the skylight opening and built a three-dimensional model, then introduced a smaller scout rover into the cave by rappelling it through the opening. The scout explored the cave’s interior while the surface team maintained a communication link.16PubMed. Cooperative robotic exploration of a planetary skylight surface and lava cave The fact that this was tested in an actual lava cave, with a real skylight geometry, rather than a lab mockup, makes the results more directly relevant to a future lunar mission.
Maintaining communication between a robot inside a tube and controllers on the surface (or in orbit) is a major challenge. Radio signals do not bend well around corners or travel through rock. One proposed solution takes inspiration from the fairy tale of Hansel and Gretel: a rover entering a tube would drop a series of small wireless communication beacons behind it as it goes, creating a mesh network that relays data back along the path like a trail of breadcrumbs. This dynamically deployed communication network approach is designed to maintain a robust link even as the rover moves deeper into irregular underground terrain.17ScienceDirect. A Hansel & Gretel breadcrumb-style dynamically deployed communication network paradigm using mesh topology for planetary subsurface exploration
Earth Analogs and What They Teach Us
Since no one has yet been inside a lunar lava tube, researchers rely heavily on terrestrial analogs to test detection methods, exploration technology, and scientific hypotheses. Several locations on Earth serve as stand-ins.
Ape Cave in Washington State, a 3.5-kilometer lava tube formed by eruptions of Mount St. Helens, has been used as a test site for gravity-based detection methods. Its length, irregular geometry, and minimal surface expression make it a useful model for simulating what a gravimeter on a lunar rover might detect. Researchers created a high-resolution 3D model of the cave and used it to generate forward gravity models under both terrestrial and lunar conditions. They then scaled the simulation to a tube five times Ape Cave’s size to approximate what lunar conditions might produce.18Journal of Geophysical Research: Planets. Gravity Modeling of Lunar Lava Tubes: Insights From a Terrestrial Analog
Lava Beds National Monument in California has served as a proving ground for ground-penetrating radar. And the lava caves of Lanzarote have hosted robotic exploration tests. In each case, the goal is not just to demonstrate a technology but to understand how cave geometry, rock composition, and environmental conditions translate from one planetary body to another. This cross-referencing between real caves and computer models is how the field slowly builds confidence that tools developed on Earth will work a quarter of a million miles away.
Lunar Tubes Compared to Martian Ones
The Moon is not the only body in the solar system with lava tubes. Mars almost certainly has them too, and comparing the three worlds where tubes are known or suspected, Earth, the Moon, and Mars, reveals how planetary conditions shape these features. Mars sits between the two extremes: its gravity is about 38% of Earth’s (compared to the Moon’s 17%), and it experienced extensive volcanism, particularly in the Tharsis region.
A comparative review found that while all three bodies likely formed tubes through similar volcanic processes, the dominant mechanisms differed. Deep inflation and thermal entrenchment dominated on the Moon, while inflation and overcrusting was more common on Mars.19ScienceDirect. Lava tubes on Earth, Moon and Mars: A review on their size and morphology revealed by comparative planetology Both Martian and lunar tubes are vastly larger than their Earth counterparts, with total volumes that can exceed a billion cubic meters. The structural integrity of these enormous lunar tubes is noteworthy: despite being orders of magnitude larger than anything on Earth, they remain within the limits of what basaltic rock under lunar gravity can support.
A broad review of lava tubes across all three worlds concluded that their structural integrity and environmental stability make them prime candidates for safe, long-term extraterrestrial habitats.20Space Science Reviews. Lava Tubes on Earth, the Moon, and Mars: Detection, Evolution, and Exploration Potential This cross-planetary perspective is useful for planning: techniques validated on Earth analogs and adapted for the Moon could eventually be applied to Mars as well, where tubes might serve a similar sheltering role for future human explorers.
Scientific Treasure Troves
Lava tubes are not just useful for shelter. They are scientifically valuable in their own right. The rocks inside a sealed tube have been shielded from the space environment for billions of years. On the lunar surface, constant bombardment by micrometeorites, cosmic rays, and solar wind gradually alters the chemistry and structure of surface rocks through a process called space weathering. Rocks inside a tube have been spared all of that. Studying the pristine, unweathered rocks exposed inside a tube could offer unique insights into the history of lunar volcanism, the composition of the Moon’s interior, and the timeline of volcanic activity.21Icarus. GRAIL gravity gradients evidence for a potential lava tube at Marius Hills on the moon
There is also the tantalizing question of volatiles. Permanently shadowed regions on the Moon are known to harbor water ice, and the interiors of lava tubes are permanently shadowed by definition. Whether significant quantities of water ice or other volatiles could accumulate inside tubes is still unknown, but the stable, cold-trap conditions make it plausible. If ice is present, it would have both scientific value, as a record of volatile delivery to the inner solar system, and practical value as a resource for future crews who could extract water for drinking, oxygen generation, and rocket propellant.
The walls of a tube also preserve a record of the lava flow that created it. Flow textures, layering, mineral crystallization patterns, and any gas bubbles trapped in the rock all tell a story about the conditions under which the tube formed: temperature, flow rate, gas content, and cooling history. For geologists, entering a lunar lava tube would be something like walking into a time capsule of the Moon’s volcanic past, one that has been sealed and untouched for roughly three billion years.
How Close Are We to Getting Inside One
Despite decades of interest, no mission has yet entered a lunar lava tube. The challenges are real: you need to identify an accessible skylight, land near it with precision, lower hardware into a vertical pit, navigate unknown terrain in total darkness, and maintain communications through rock. Each of these problems has partial solutions, but assembling them into a funded, flight-ready mission remains a work in progress.
Several space agencies and commercial entities have expressed interest. ESA has funded lava-tube exploration technology development, and the cooperative robotic exploration concept tested in Lanzarote was part of that effort.22PubMed. Cooperative robotic exploration of a planetary skylight surface and lava cave NASA’s interest in the Marius Hills skylight as a potential exploration target dates back years. Japan’s SELENE mission first confirmed the Marius Hills Hole as a genuine pit, and subsequent studies have kept it near the top of the list of scientifically compelling lunar destinations.
The pieces are moving into place: high-resolution mapping of skylights, gravity-based tube detection, radar and magnetometry for subsurface characterization, robotic systems that can rappel into caves, and communication networks that work underground. What is missing is the mission itself, a dedicated flight that ties these technologies together and drops a robot into the dark. Given the pace of lunar exploration in the 2020s, with multiple countries and companies sending landers to the Moon, it may not be long before someone takes that step.