The physical properties of mercury turn this simple fountain into a mesmerizing crucible of liquid metal. Calder's 1937 sculpture commemorates the siege of Almadén, Spain, known for its mercury mines, and is now part of the collection at the Fundació Joan Miró. Atlas Obscura, which features more photos of the fountain here, tells the backstory:

For many years, the world's greatest source of mercury were the mines at Almadén, Spain, which produced some 250,000 metric tons of mercury over nearly two millennia of operation. So when Spain decided to build a monument to the mine (which was long worked by criminals and slave laborers, most of whom died of mercury poisoning), they commissioned American sculptor Alexander Calder to build a graceful fountain which, instead of water, would pump pure mercury. It was to be displayed at the 1937 World's Fair.

Mercury is the only metal that is liquid at room temperature. This video from mercuremiro on YouTube shows the fountain in action.

Via Atlas Obscura.