The newest lake in Helsinki isn’t on the map—it’s more than 300 feet below the city, and it’s the latest cog in a centralized cooling, heating, and energy-production system that’s propelling the city closer to its goal of running on 100 percent carbon-neutral energy by 2050.

Seawater cooled in the reservoir is piped through the inner city, creating a cheaper, greener, and more efficient form of air conditioning for some 300 connected buildings. While district temperature regulation like this isn’t new, the way Helsinki has joined it with multiple systems is a pioneering step. The city collects the energy that escapes through the process and recycles it. According to Kosti Koski, head of district cooling at energy company Helen: “The next big thing will be on the heating side, where we are transferring our production to carbon-neutral fuels.” helen.fi