Geothermal is too expensive, but Dig Energy’s impossibly small drill rig might fix that

On a farm near Manchester, New Hampshire, I was recently treated to a gusher of dirty water, not exactly the sort of thing that most startups will show a reporter. But for Dig Energy, the mud is a feature, not a bug, of its compact drilling rig. 

The startup, which has been operating in stealth for the last five years, developed the water-jet drilling rig in an effort to make geothermal heating and cooling so inexpensive that it will displace fossil fuel boilers and furnaces. The rig is central to that, promising to slash drilling costs by up to…