The war in Ukraine has forced a rethink of where we get our energy from as Europe tries to wean itself off Russian gas. But could super-sized heat pumps help to heat thousands of homes and businesses? Two huge schemes are about to be switched on in Gateshead and London - and the hope is they could provide a greener and cheaper source of warmth.
"Coal mining was massive in the North East," says Jim Gillon, walking across a building site in Gateshead.
"And where we're standing there are six different mine workings beneath our feet."
Jim is the Energy Services Manager for Gateshead Council - and he's giving this former fossil fuel site a green makeover for an ambitious new heating scheme.
He points to a borehole that descends 150m beneath the muddy earth. Like many old coal mines, it's now flooded with water. But the water is naturally warm at 15C - and this heat is key.