There is a massive battery right under your feet. Unlike a flammable lithium ion battery, though, this one is perfectly stable, free to use, and ripe for sustainable exploitation: the Earth itself. While temperatures above ground fluctuate throughout the year, the ground stays a stable temperature, meaning that it is humming with geothermal energy. "Every building sits on a thermal asset," said Cameron Best, director of business development at Brightcore Energy in New York, which deploys geothermal systems. "I really don't think there's any more efficient or better way to heat and cool our homes." A couple of months ago Eversource Energy commissioned the US's first networked geothermal neighbourhood operated by a utility, in Framingham, Massachusetts. Pipes run down boreholes 600-700ft (about 180-215 metres) deep, where the temperature of the rock is consistently 55F (13C). A mixture of water and propylene glycol ... pumps through the piping, absorbing that geothermal energy. Heat pumps use the liquid to either heat or cool a space. If deployed across the country, these geothermal systems could go a long way in helping decarbonise buildings, which are responsible for about a third of total greenhouse gas emissions in the US. Once a system is in place, buildings can draw heat from water pumped from below their foundations, instead of burning natural gas. The networks ... can be set up almost anywhere.