Archaeologists studying an ancient mosaic found by workers laying cable south of Rome have been astonished to discover that it is an optical illusion.

Viewed one way up it is a bald old man with a beard, but turned the other way round it is a beardless youth.

Roberto Cereghino, a government archaeological official, told the Italian daily newspaper Corriere della Sera that it was "a very beautiful work, of immense significance".

He said it appeared to be a depiction of Bacchus.

The double face is surrounded by objects that were used in Bacchanalian rites: an ancient musical instrument, the sistrum, a two-handed drinking bowl, and a priestly wand. The mosaic's optical trickery may be linked to the fact that Bacchus was the god of wine.

Mosaics containing optical illusions have been found in north Africa, but this is thought to be the first discovery of such a work in Italy.

The double head was unearthed last month in an industrial area near the town of Pomezia that was previously thought to have been thoroughly explored for archaeological remains.

The mosaic has since been removed from the site for restoration, and there are plans to put it on display in Rome later this year.