 The concept of the pilae stacks is that the floor is constructed at an elevated position, allowing air to freely circulate underneath and up, through the hollow bricks, into the structure walls.Examples of such baths are found not only in Rome, but also in distant parts of the Roman Empire such as Roman Britain, or Chellah, in modern-day Morocco.This architectural technique was the first form of underfloor heating and the same principle is still used today.