Colonising the Asteroids is typically imagined as either living in domes on the surface and/or living in excavated tunnels beneath. However, as our knowledge of asteroids has grown, a new location has become apparent – inside the natural cavities that (might) exist within the asteroids. Prior to extensive asteroid studies over the last 20 years, asteroids were conceived of as solid bodies, but as the masses and sizes of the asteroids have been measured by telescopic and radar studies of their shapes, mutual interactions, space-probe flybys and even the orbits of their moons, we’ve come to realise that many asteroids are very “porous”. Not the 1%-5% porosity we know of in soil, but much greater proportions of asteroid insides seem to be just empty space. What if we could use that space?