Looks like Mars and the moon contain huge lava tubes that offer protection from solar radiation and meteors—which makes them possible homes for future explorers, LiveScience reports. A new paper says Martian tunnels appear to range from 130 to 1,300 feet in diameter, while the moon's are 1,600 to 3,000 feet and reach such heights that the world's tallest building, Dubai's 2,720-foot Burj Khalifa, could fit inside. "Tubes as wide as these can be longer than 40 kilometres, making the moon an extraordinary target for subsurface exploration and potential settlement in the wide protected and stable environments of lava tubes," study co-author Riccardo Pozzobon says in a press release. "The latter are so big they can contain Padua's entire city center."
The above lava-tube sizes are really estimates, based on 3D laser scans of similar tubes on Earth as compared to satellite images of lunar and Martian tubes. The Martian ones have more likely collapsed, researchers say, but many lunar tunnels should still be standing. So how to go spelunking on the moon? Such rovers don't exist yet, but Sci-Tech Daily reports that the European Space Agency is training astronauts in cave hiking and soliciting ideas for lunar-cave exploration. "Space agencies are now interested in planetary caves and lava tubes, as they represent a first step towards future explorations of the lunar surface and towards finding life (past or present) in Mars' subsurface," says co-author Francesco Sauro. (Meanwhile, researchers have revised the age of the moon.)