Mercury fountain
The Mercury fountain is a type of fountain constructed for use with mercury rather than water.
The most noted example is a modern sculpture designed by the American artist Alexander Calder and commissioned by the Spanish Republican government for the 1937 World Exhibition in Paris. The artwork is a memorial to the siege of Almadén, which then supplied 60% of the world's mercury, by General Franco's troops. A direct counterpart is Picasso's Guernica. Calder's mercury fountain, now at the Fundació Joan Miró in Barcelona, is displayed behind glass for safety reasons.
Background
History of mercury fountains
Mercury fountains existed in some castles in Islamic Spain; the most famous one was situated at the Kasr-al-Kholaifa in Córdoba.
This is probobly the coolest thing you'll ever see if you were to actually see it.