If humans are going to live someplace other than Earth, the moon seems like a good option. It’s nearby and could serve as a launchpad for missions to other locations in the solar system. Scientists have long wondered if shadowy crevasses on the lunar surface could be entrances to caverns that could serve as a home for future colonists. New data from Japanese and US researchers indicates such caverns do exist.

The moon is a cold, lifeless chunk of rock today, but it was active in the past when the interior was still cooling. Geological activity like lava flows and volcanic activity could have carved out tubes and caves similar to features we have here on Earth. The international team examined several dark pits on the moon in what is known as the Marius Hill region. These features are considered some of the best candidates to be the openings to ancient lava tubes.

Researchers from the Japanese space agency JAXA analyzed radar data from the SELENE spacecraft, which was deployed in 2007 to study the moon’s geology. It turns out the radar instruments on SELENE were also able to provide good evidence for the existence of lava tubes. Scientists fired multiple radar bursts into the suspected lava tubes, and what they got back was a two-part echo indicating the interior of the pit has a floor and a ceiling. Several other sites produced the same echo patterns, so there may be multiple tubes in the region.

The same area surveyed by JAXA also happens to be where NASA conducted studies with a mission called GRAIL (Gravity Recovery and Interior Laboratory). This mission used a microwave band sensor system to map the gravitational field of the moon. This data can also tell you where there’s a mass deficit–areas where there’s less mass than the surrounding space. Combining SELENE and GRAIL data, the suspected lava tubes show mass deficits. The team was even able to use the combined data to estimate the size of the chamber. The underground cavern is several kilometers long and at least one kilometer tall.

A massive space like this could serve as a location to study the internal structure of the moon, as well as build a colony. Without an atmosphere or magnetic field, the moon’s surface doesn’t offer any protection from radiation. Any extended stay on the moon (or Mars, for that matter) would require heavy shielding to prevent excessive radiation exposure, but you don’t need to build something on the surface to do that when the moon’s regolith does the job fine when you’re underground. There’s a lot of hype about colonizing Mars, but the moon is looking like an even more attractive place to start.