Mona Lisa keeps smiling through the heat thanks to Paris's best kept secret

The underground cooling system snakes beneath Parisians' feet 

The Louvre museum. (AP Photo/Lewis Joly)

The Mona Lisa may maintains her famously enigmatic smile because she benefits from one of Paris's best-kept secrets: an underground cooling system that has helped the Louvre cope with the sweltering heat that has broken temperature records across Europe.

The little-known "urban cold" network snakes beneath unsuspecting Parisians' feet at a depth of up to 98ft (30m), pumping icy water through 55 miles (89km) of labyrinthine pipes, which is used to chill the air in more than 700 sites.