Mercury, also known as quicksilver, is a beautiful, mirror-skinned metal that is liquid at room temperature. Unfortunately, it is also extremely toxic.
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.
At the time, the inherent beauty of the liquid metal was well understood, but its toxicity was not. Today, the fountain resides in the Fundacio Joan Miro in Barcelona, Spain, and continues to pump pure mercury, though now it does so behind a pane of glass to protect viewers from touching or breathing fumes produced by this deadly work of art.