Mole hills

The ventilation works through that hills, too, for they are not pressed and the air can circulate. They have exits, but the also get out of these hills sometimes. The new ones, I guess, were built after some of the older ones get pressed or deleted because of the weather or some of them were even frozen.

Hi Emma, they build tunnels under the earth and the hills are on the one hand the earth from behind, on the other side it keeps enemys away and it helps to regulate the climate and keep dirt out of the tunnels. more details can be found here: http://en.wikipedia.org/wiki/European_Mole