A lava tube is a natural tunnel formed when the outer surface of a flowing lava stream cools and hardens into a solid roof while molten lava continues to move beneath it. Once the eruption slows and the lava drains away, a hollow cave-like passage remains. These tubes range from narrow crawl spaces to massive corridors hundreds of meters wide, and they exist on every continent with volcanic activity, as well as on the Moon, Mars, and likely Venus.
How Lava Tubes Form
Lava tubes develop through several related processes, but they all start the same way: hot, fluid lava flows downhill and begins losing heat from its exposed surface. As the top layer cools below its solidification temperature, a thin crust forms. This crust acts as insulation, trapping heat inside and allowing the lava beneath to stay liquid and keep flowing. Over time, the crust thickens into a stable roof.
The specifics vary depending on the terrain. In confined channels, the crust often grows inward from the edges, gradually extending toward the center until the two sides meet and seal the flow inside a tube. In other cases, lava overflows its banks and builds up arched walls that eventually connect overhead. A third process involves free-flowing lava lobes that simply extend forward beneath their own solidifying skin, inflating as more lava pushes in from behind. A fourth mechanism occurs when floating plates of cooled crust drift downstream and merge together into a continuous roof.
Research on Mount Etna’s lava channels showed that crust formation depends on a balance between the mechanical stress of the flowing lava and the strength of the cooling surface layer. Wider channels and gentler slopes both reduce the forces trying to break up the crust, which means a larger fraction of the channel gets covered. Narrow, steep channels tend to keep their surfaces exposed longer because the fast-moving lava tears the crust apart before it can solidify.
Once a tube is fully enclosed, it becomes remarkably efficient at transporting lava over long distances. The insulating roof reduces heat loss so dramatically that lava can travel several kilometers from the volcanic vent without cooling significantly. This is why some of the longest lava flows on Earth were fed by tube systems rather than open surface channels.
What the Inside Looks Like
Walking into a drained lava tube is like entering a rough, organic version of a subway tunnel. The walls are typically dark basalt, and the floor may be smooth, ropy, or littered with rubble from partial ceiling collapses. The cross-section is usually wider than it is tall, though shapes vary widely depending on how the tube formed and how many times lava flowed through it.
Several distinctive features mark the interior. Horizontal lines along the walls, sometimes called “bathtub rings,” record different levels at which the lava surface stood as the flow rose and fell over time. Dripping formations develop as the lava level drops, similar in shape to stalactites but made of solidite basalt rather than mineral deposits. These lava drips (sometimes called lavacicles) freeze in place as the tube cools. You may also see shelves along the walls where lava crusted at a high-water mark, then the flow beneath it drained away.
Some tubes contain multiple levels, formed when a second flow entered an existing tube and partially filled it, creating a new floor above the original one. Others branch into side passages or dead-end chambers where lava pooled and solidified.
Skylights and Roof Collapse
The openings you see in photographs of lava tubes, circular holes in the ground that reveal the darkness below, are called skylights. They form when a section of the roof becomes too thin or too fractured to support itself and collapses inward. On Earth, this happens through weathering, earthquakes, and erosion over thousands of years. On the Moon and Mars, meteoroid impacts and shallow seismic activity can punch through the ceiling, along with pre-existing fractures in the rock.
Skylights serve as the primary way scientists locate lava tubes, both on Earth and on other planets. From above, an intact tube is invisible, but a skylight shows up clearly in satellite images as a dark pit in an otherwise smooth lava field.
Life in Total Darkness
Despite having no sunlight, lava tubes host surprisingly active microbial ecosystems. The caves maintain stable temperatures and humidity year-round, creating conditions that certain microorganisms thrive in. Research inside the Cueva del Viento lava tube system in Tenerife, one of the longest volcanic caves in the world, revealed white microbial mats coating the walls. These mats contain bacteria that break down urea into ammonia and carbon dioxide, while other species handle nitrogen cycling, converting nitrites and fixing carbon dioxide to produce energy.
The foundation of these underground food webs is chemolithoautotrophic microorganisms, bacteria that derive energy from chemical reactions with minerals in the rock rather than from sunlight. They serve as the primary producers in the tube ecosystem, generating organic matter that supports a secondary community of organisms. Some of these secondary microbes are capable of breaking down complex plant and animal material that washes in from the surface, while others specialize in degrading specific chemical compounds. The result is a self-sustaining microbial community that operates entirely independently of photosynthesis.
Notable Lava Tubes on Earth
The longest known lava tube on Earth is Kazumura Cave on the Big Island of Hawaii, stretching over 65 kilometers with a vertical drop of more than 1,100 meters. Hawaii’s shield volcanoes produce the fluid, low-viscosity lava that forms the most extensive tube systems. Hawaiʻi Volcanoes National Park contains several accessible tubes, including the well-known Thurston Lava Tube (Nāhuku), which visitors can walk through.
Other significant tubes include the Cueva del Viento system in Tenerife (over 17 kilometers of mapped passages), Manjanggul Cave in South Korea, and extensive tube networks in Iceland, the Canary Islands, and the lava fields of the Pacific Northwest in the United States. Lava Beds National Monument in northern California alone contains over 700 caves, most of them lava tubes.
Lava Tubes on the Moon and Mars
Lava tubes exist on other worlds too, and they tend to be far larger than anything on Earth. Under the Moon’s lower gravity and lack of atmosphere, lava channels and tubes form at least ten times larger in each dimension. That translates to passages potentially hundreds of meters wide, over a hundred meters deep, and tens of kilometers long.
This scale makes lunar lava tubes serious candidates for future human habitats. A NASA radiation safety analysis found that the basalt rock surrounding a lava tube blocks virtually all dangerous space radiation. Galactic cosmic rays, the most penetrating form of space radiation, produce no detectable effects below about 6 meters of rock depth. Solar particle events, which are intense but less penetrating, are blocked by less than 1 meter of basalt. Even shallow tubes with roofs only 1 to 2 meters thick would keep radiation doses well below the safety limits set for astronauts on a monthly, annual, and career basis.
Beyond radiation, lava tubes offer protection from meteoroid impacts, micrometeorite bombardment, and the extreme temperature swings on the lunar surface. Inside a tube, temperatures hold nearly constant at around minus 20°C, compared to surface conditions that swing from roughly 127°C in direct sunlight to minus 173°C in shadow. A stable, predictable environment like that dramatically simplifies the engineering needed for a long-term base.
How Scientists Find Hidden Tubes
On Earth, lava tubes are often discovered the straightforward way: someone notices a skylight or a cave entrance in a lava field. But finding tubes that are still sealed beneath the surface requires more sophisticated tools. Ground-penetrating radar can detect voids below the surface, and gravity surveys (gravimetry) can identify the subtle density differences that an empty tube creates in the rock above it.
On other planets, the challenge is bigger. Scientists use synthetic aperture radar (SAR) from orbiting spacecraft to peer into skylights and map the geometry of the tube beyond the opening. Radar waves from a side-looking sensor travel through a skylight and bounce off the tube’s interior walls, revealing the shape and size of the passage below. This technique has been applied to lava tubes on Earth (including the Corona lava tube system in the Canary Islands, mapped using a combination of spaceborne radar, ground-based lidar scans, and drone photogrammetry) and is now being used on Venus with data from NASA’s Magellan spacecraft.
Recent analysis of Magellan radar images identified evidence of a lava tube on Venus by looking for surface collapses consistent with skylights. The Magellan data has a resolution of 75 meters per pixel, which likely means many smaller collapse features were missed. Upcoming missions to Venus will carry radar systems with resolutions of 15 to 30 meters, along with a subsurface radar sounder capable of penetrating several hundred meters below the surface, opening the door to a much more complete survey of Venusian lava tubes.