A mercury fountain is a fountain constructed for use with mercury rather than water.
Mercury fountains existed in some castles in Islamic Spain; the most famous one was located at the Kasr-al-Kholaifa in Córdoba.
Calder's Mercury Fountain
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Calder's fountain of mercury at the Fundació Joan Miró in Barcelona
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 percent 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.