In a city filled with ancient treasures and relics, some are calling it the Italian capital's biggest and best kept secret. Now, the public can finally see what's thought to be the largest ever fresco and mosaic complex of ancient Rome.

Authorities have unveiled a first century A.D. depiction of a maritime city in what became the largest bath complex in the world - the Trajan Baths, the site of a new museum inaugurated on Wednesday.

The “Painted City” fresco and “Great Mosaic” were discovered in an underground gallery during excavations in the 1990s of the Baths of Trajan on the Oppian Hill near the Colosseum.

After being hidden underground for centuries, its blues and terracotta colors are still clear but faded.

Massive monuments