To look at, the dark, dripping sewers of Brussels seem an unlikely place for anything particularly valuable to be hidden. But a wet day reveals all.

During a winter downpour, the brick tunnels become subterranean waterslides. Fresh rain tumbles from drains in the street above, joining waste water already in the sewers from sinks, baths, showers and toilets on their long journey downstream. The volume of these fluids and, crucially, their temperature are the reason that the city’s energy experts’ minds are in the gutter.

“The heat of the tunnels always astonished me,” says Olivier Broers, head of investment at the city’s water company, Vivaqua. He first noticed the Belgian city’s dormant heat source 20 years ago when he worked in tunnel restoration. He recalls days when there was ice and snow in the city, but on climbing down a manhole, he would find the sewers an ambient 12-15C. “Enough to fog my glasses,” he recalls.