hypocaust

This Roman bathhouse floor is standing on a hundred tiny brick pillars.

It's called a hypocaust, Greek for burnt from below: a mosaic floor raised over a hollow chamber.

A furnace outside sends hot air between the brick stacks, called pilae, warming the heavy stone slab.

Then the heat rises through hollow clay flues inside the walls, wrapping the whole room in warmth without smoke.

From Roman baths in Italy to frontier villas in Britain, it kept stone rooms warm for centuries.

Its cousins: the ondol in Korea, the kang in China, and the gloria in Spain.

Hypocaust. A floor raised on pillars to let fire flow underneath.