I magine a habitable colony on Mars or the Moon and the kinds of structures that come to mind are probably gleaming domes or shiny metallic tubes snaking over the surface. But with no Earth-like atmosphere or magnetic field to repel solar radiation and micrometeorites, space colonists would probably need to pile metres-thick rocks and geological rubble onto the roofs of such off-world settlements. More like a hobbit hole than Moonbase Alpha.
There could be another solution, however, that would offer future colonists safer and far more expansive living space than any cramped base built on the surface. Writing in Acta Astronautica, Raymond Martin, an engineer at Blue Origin, a rocket company, and Haym Benaroya, an aerospace engineer at Rutgers University, explore the benefits of setting up a Moon base inside giant geological tunnels that lie just below the lunar surface.
First discovered during the Apollo programme, these lunar lava tubes are a legacy of when Earth’s nearest celestial neighbour was geologically hyperactive, with streams of boiling basaltic magma bursting from the interior to flow across the Moon’s surface as lava. Found on Earth (see picture), and identified on Mars, lava tubes form when the sluggish top layer of a lava stream slows and cools, forming a thick and rocky lid that is left behind when the rest of the lava underneath eventually drains away.