They course through many of the world’s biggest cities: miles of underground pipes built decades ago, ferrying steam or hot water to a network of buildings. Misty manhole covers on urban streets signal the presence of what’s known as district energy.

Widely used but rarely mentioned in conversations about how cities can slash climate-warming greenhouse gases while sustaining growth, district energy is attracting new interest. The United States alone has more than 700 of these subterranean systems, some dating back to the 1880s.

They’ve often been attached to coal- or oil-fired power plants, piping steam or water to a circuit of buildings for heating and cooling. Now many old systems are getting retrofits to deliver “green steam” generated with cleaner fuels