Finland isn’t exactly known for hot weather. But as the climate changes and summer heat waves become more common in northern Europe, don’t expect to see more air conditioners in Helsinki: The city is pioneering a huge cooling system that uses cold water from nearby lakes and the sea instead of electricity.

Hundreds of feet underneath an ordinary-looking park in downtown Helsinki, a local energy company built a huge reservoir filled with nearly 9 million gallons of lake water. When the system is fully operational next summer, the water will be pumped to local buildings in the area to keep them cool. At night, the water will flow back underground, where waste energy will be used to cool it down again.





The tank is the latest piece of the city’s quickly growing cooling network, which already uses seawater to keep buildings comfortable. 300 buildings–mostly offices and commercial spaces–are connected to the network now, and as summers get hotter, the city plans to keep expanding the system.

“There are over 10,000 potential cooling customers in Helsinki,” says Seija Uusitalo, a spokesperson for Helsinki Energy. “Demand for district cooling is growing rapidly, and we are here to satisfy the demand for high efficiency, environmentally friendly cooling and heating solutions. Every year, we’re building several kilometers of new district cooling.”

The system is five times more efficient than hooking up buildings with conventional air conditioners, and can cut carbon pollution by 80%. Because it works at a neighborhood scale, rather than with individual buildings, it’s a way for the city to reduce emissions quickly and easily.

It’s not something that could work everywhere. Helsinki’s geography, with a thick layer of bedrock just under the ground, has made the city uniquely suited to build state-of-the-art infrastructure underground–from data centers and hidden roads to the world’s largest heat pump, which keeps the city warm during colder months.

But Helsinki hopes to inspire others to follow its example to save massive amounts of energy. “There might be huge potential in other cities for this kind of system,” says Uusitalo