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By Thomas Adamson and Nico Garriga

The Associated Press

PARIS 鈥� The Mona Lisa may maintain her famously enigmatic smile because she benefits from one of Paris鈥� best-kept secrets: an underground cooling system that鈥檚 helped the Louvre cope with this summer鈥檚 sweltering, record-breaking heat.

The little-known 鈥渦rban cold鈥� network snakes beneath Parisians鈥� feet at a depth of up to 98 feet, pumping icy water through 55 miles of labyrinthine pipes. That water is used to chill the air in over 700 sites. The system, which uses electricity generated by renewable sources, is the largest in Europe 鈥� and chugs on around the clock with a deafening noise totally inaudible above ground.

Paris City Hall has now signed an ambitious contract to triple the size of the network by 2042 to 157 miles. That would make it the largest urban cooling system in the world. The new contract is intended to help the city both adapt to and combat the threat of climate change. Many parts of Europe hit 104 degrees Fahrenheit in July.

The city is extending the cooling network to hospitals, schools and metro stations over the next two decades. It鈥檚 unclear how much of the system will be operational by the time Paris hosts the Olympic Games in 2024, but it鈥檚 possible the systems will be used at several Olympic sites.

Unbeknownst to millions of tourists, the piping currently cools the City of Light鈥檚 most emblematic sites, such as the Louvre and the Quai Branly Museum. It might even help cool the tempers of agitated lawmakers as it is used to drop temperatures in the National Assembly.

The scheme is operated by joint-venture company Fraicheur de Paris 鈥� 85 percent owned by state French energy company EDF and the rest by public transport operator RATP. The company鈥檚 officials tout its benefits for the entire French capital.

鈥淚f all (Parisian) buildings get equipped with autonomous installations (such as air-conditioning), it will gradually create a very significant urban 鈥榟eat island鈥� effect,鈥� said Maggie Schelfhaut of Fraicheur de Paris, referring to the increased heat in cities due to less vegetation, which cools, and more urban infrastructure, which absorbs the sun鈥檚 rays.

But the pipe network could make the whole of Paris 1.8 degrees Fahrenheit cooler than if autonomous installations were put up across the city, Schelfhaut said.

鈥淥ne degree (Celsius) less in the city center is a lot,鈥� she added.

Three of the 10 high-tech cooling sites lie on the Seine River and are accessible by a retractable spiral staircase barely visible from street level 鈥� in something resembling the lair of the 鈥淭eenage Mutant Ninja Turtles.鈥�

When the Seine鈥檚 water is cold enough, a machine captures it and uses it to chill the system鈥檚 water. The heat created as a byproduct is sent back into the Seine, where it is absorbed. The chilled water is then pumped though the system鈥檚 pipes to its 730 Parisian clients.

Paris鈥� cooling sites all use renewable energy sources such as wind turbines and solar panels. Four new solar energy sites that will feed into this network are also earmarked for construction. French officials see this energy independence as particularly important given the threat of Russia聽cutting off energy supplies to Europe.

Russian energy corporation Gazprom recently聽cut the amount of natural gas flowing through a major pipeline聽from Russia to Europe to 20 percent of its capacity. European nations are聽rushing to find alternatives聽amid fears that Russia could completely cut off gas exports 鈥� used for industrial purposes, to generate electricity and to cool homes 鈥� to try to gain political leverage over the bloc.

The merits of using a cooling system that uses renewable energy to operate are already being felt by sites that use them. The world鈥檚 most visited museum, the Louvre, has benefited from the network since the 1990s 鈥� with officials proud of its ecological, economic and art conservation advantages.

鈥淚t allows us to benefit from energy with a lower carbon footprint available all year round,鈥� said Laurent Le Guedart, the Louvre鈥檚 heritage director. 鈥淭he particularity of the Louvre Museum is that it needs to use iced-cold water to correctly conserve the artwork and to control the humidity.鈥�

The Louvre does not use air-conditioning and officials say the cooling also wins them much needed floor space in the sprawling, yet cramped, former palace that is home to 550,000 artworks.

The system is a money-saver given the rising cost of energy linked to the聽Ukraine conflict, Le Guedart said. It operates notably in the State Room of the Pavillon Denon where the Mona Lisa lives. Perhaps it鈥檚 why beads of sweat have never trickled down the brow painted by Leonardo da Vinci.

鈥淭he energy bill of the Louvre is around 10 million euros per year in 2021,鈥� Le Guedart said. 鈥淲e are trying to control this bill as much as possible, amid the evident fluctuations and increases to energy costs.鈥�

As Russia continues to roil the energy market, the cooling system could cushion the shock to Louvre operators and others.

Editor鈥檚 note: Jade Le Deley in Paris contributed to this report.