The construction of this large hall with a hemispherical vault imitating that of the Pantheon was completed in 1779 according to a project of Michelangelo Simonetti. The walls carry a series of niches for displaying colossal statues, between which are half-columns which support outsize busts. The floor is an amazing 18th century assemblage of mosaics from the first decades of the 3rd century A.D. which were found at Otricoli and at Sacrofano. At the centre of the room is a huge red porphyry basin which has a circumference of 13 metres. The basin must once have embellished one of the large public spaces of imperial Rome.