THE WOODLANDS, Texas—A system of buried, empty lava tubes hides beneath the moon’s surface, remnants of a bygone age when the volcanically active moon launched fountains of fire into space.

At least, that’s what scientists think.

For years, teams have hunted for these elusive sublunar tunnels, which can be large and sturdy enough to house entire cities. In fact, lunar lava tubes could be ideal locations to establish a moon base, as their thick roofs would shield humans from harmful radiation and small meteorite impacts. But until now, the strongest observational hints of the tubes’ existence came from a smattering of detectable surface features, including skylights and rilles, channel-like depressions thought to form when tubes collapse.

This week, scientists announced that the