“The long, 190-ft sunken area of the Big Dig tunnel is no little pot hole and the length indicates a large mass of water,” Jaroszewicz says. When freezing 130 ft below the surface, you are likely to find many different aquifers that could freeze, he says.

“There are lots of aquifers near the ocean,” he says. “When you are deep like that the ground is potentially 40° F, only 8° above freezing, so there is not much temperature differential for every pound of water.”