Mostly its good drainage. Some build them so their nests provide air pockets. Some close the entrances to their nest (with their bodies) for some time. In extre

Mostly its good drainage. Some build them so their nests provide air pockets. Some close the entrances to their nest (with their bodies) for some time. In extreme cases they evacuate to higher ground. They are extremely fast with the last. I can study that everytime I mow the lawn. Interestingly the species in my garden 'Lasius flavus' often seem to care their eggs above ground in the hill, not underground. Whereas I never found eggs above ground in nests of 'Lasius niger'. But then again, I'm no expert in ants and its purely anecdotal. Just curious.