The double-mirror solar furnace in Mont-Louis, France.
Constructed in 1949 in the citadel of Mont-Louis in the French Pyrenees, this double-mirror solar furnace concentrated the sun’s rays to melt just about anything that wandered into its 3,000 °C beam. Engineer Félix Trombe designed the solar furnace and used it for high-temperature experiments on materials and to demonstrate how to fire ceramics without using wood.
GIF via Archillect.