In terms of mapping, yes: we have more detailed surface maps of the Moon than we do of Earth’s ocean floor. Only about 27% of the global seabed has been mapped to modern standards, while the Moon’s surface has been mapped at a resolution of meters in many areas. That gap sounds counterintuitive, since the ocean is right here on our planet, but it comes down to physics. The tools that work brilliantly in space are nearly useless underwater.
The Mapping Gap in Numbers
NASA’s Jet Propulsion Laboratory puts it bluntly: there are better maps of the Moon’s surface than of the bottom of Earth’s ocean. Satellites and orbital instruments have mapped the lunar surface at meter-scale resolution across most of its area. The best global picture of the ocean floor, by contrast, is a grid with cells roughly 1 by 1 kilometer, and even that relies heavily on indirect measurements from satellites rather than direct sonar surveys.
Ships equipped with sonar can produce incredibly detailed seafloor measurements, but as of mid-2025, only 27.3% of the ocean floor has been surveyed this way. The GEBCO Seabed 2030 project, an international effort to map the entire seabed by the end of this decade, recently added four million square kilometers of new data, an area roughly the size of the Indian subcontinent. That sounds enormous, and it is, but it still leaves nearly three-quarters of the ocean floor essentially unseen at useful resolution.
Why the Ocean Is Harder to Map Than the Moon
The core problem is that the remote sensing technology we use everywhere else, electromagnetic waves like radio and light, falls apart in seawater. Saltwater’s high electrical conductivity causes radio signals to lose energy almost immediately. Signals above 30 MHz are absorbed within centimeters. Even low-frequency signals below 1 MHz only penetrate a few tens of centimeters before fading, and they require impractically large antennas.
The Moon has no such barrier. It sits in a near-perfect vacuum, with roughly one million billion times fewer molecules per cubic centimeter than Earth’s atmosphere. Electromagnetic waves travel from orbiting spacecraft to the lunar surface and back with virtually no interference. A satellite can map the Moon from orbit the same way it maps dry land on Earth. The ocean blocks that entire approach.
Instead, ocean mapping relies on sonar: ships send sound pulses toward the seabed and measure the echoes. This works well but is painfully slow. A single survey vessel can only cover a narrow strip of seafloor at a time, and the ocean spans 361 million square kilometers. At current rates, mapping it all ship by ship would take decades and cost billions.
Pressure vs. Vacuum
Exploring the deep ocean in person is also far more dangerous than visiting the Moon, in a purely physical sense. The lunar surface is a vacuum, which means spacecraft need to keep about one atmosphere of pressure inside. That’s a well-understood engineering problem solved in the 1960s. The deep ocean flips the challenge: at the bottom of the Mariana Trench, roughly 11 kilometers down, the water exerts pressure exceeding 1,000 atmospheres. Building a vessel that won’t be crushed under that load is a much harder materials problem.
The numbers reflect this. Twelve people have walked on the Moon. Only six have ever reached the bottom of the Mariana Trench in submersibles, and none of them walked on it. Deep-ocean vehicles are expensive, highly specialized machines that take years to build and test, and each dive is a high-stakes operation.
What We Don’t Know About Ocean Life
The mapping gap is only part of the story. Biologically, the ocean is far less understood than its sheer proximity would suggest. Scientists estimate that only about 10% of marine species have been documented so far. The Ocean Census, a global initiative launched in recent years, has already identified over 850 new species and aims to catalog 100,000 more within the next decade.
The Moon, by comparison, has no biology to discover. Its surface chemistry and geology are relatively simple: rocks, dust, and craters in a vacuum. That simplicity makes it easier to study comprehensively. The ocean is a living system with complex chemistry, currents, pressure zones, hydrothermal vents, and ecosystems that change with depth, temperature, and geography. Each of those dimensions adds layers of unknowns that the Moon simply doesn’t have.
Why the Gap Exists
It’s not that scientists are uninterested in the ocean. The gap exists because of how funding and technology have historically aligned. Space exploration benefited from Cold War competition and massive government investment, which produced the satellites and instruments that mapped the Moon, Mars, and beyond. Ocean science has never received comparable sustained funding, in part because its discoveries are less visible to the public and harder to frame as geopolitical achievements.
There’s also a practical asymmetry. A single orbiting spacecraft can map an entire planetary surface over weeks or months. No equivalent shortcut exists for the ocean floor. Every improvement in seafloor coverage requires more ship time, more sonar equipment, or new satellite techniques that measure the ocean surface to infer what lies beneath, a method that sacrifices resolution for speed.
So the popular claim holds up, with a caveat. We have mapped the Moon’s surface in finer detail than the ocean floor, and we’ve sent more people to the lunar surface than to the deepest ocean trenches. But “knowing more” depends on what you’re measuring. We understand the Moon’s geology well because there’s less to understand. The ocean’s complexity means that even after we map every square kilometer of seabed, cataloging what lives there and how it functions will take far longer.