Helsinki, Finland, isn’t known for especially hot weather, but during the short summer season, the city has an unusual cooling system—an underground lake.
The buildings in the city, Gizmodo reports, are cooled by a man-made lake that stores nine million gallons of ice cold water. It's in a cavern that is 300 feet long and 300 feet beneath the Esplanade Park in the center of the city.
When the weather gets warm, the cold water is pumped out into buildings in Helsinki, where the frigid water is used instead of standard air conditioning.
The city says that compared to the environmental impact from conventional air conditioning systems needed to cool the same number of buildings, their centralized cooling system is emitting about 80 percent less carbon dioxide and other greenhouse gases. Based on energy demand, the city also estimates that the overall energy efficiency is about five times that of a building-specific cooling systems.
There are other district cooling systems around the world. In Toronto, buildings like the Ritz and Mount Sinai Hospital are cooled by water taken from the icy depths of Lake Ontario. In Ithaca, N.Y., Cornell University has a similar system, although it depends on a natural lake and hasn't been without controversy.