Several years ago I worked as a groundskeeper at the Birch Hill Cemetery here is Fairbanks. (Under-employed artists end up working all sorts of interesting jobs.) I was removing some rotten boards around one of the graves and uncovered these ant high-rise tunnels. The tunnels were only on the south side of the grave where they could absorb the most warm from the sun. They may have been dug for warmth, but I thought the tunnels looked pretty cool.
All over the cemetery the ants took advantage of human structures: tunneling along the inner side of boards, or building their nests against the south sides of headstones and grave covers to take advantage of the thermal mass of the stone and concrete.