Ostia Antica is a large archaeological site, close to the modern town of Ostia, that is the location of the harbour city of ancient Rome, 15 miles (25 kilometres) southwest of Rome. "Ostia" (plur. of "ostium") is a derivation of "os", the Latin word for "mouth". At the mouth of the River Tiber, Ostia was Rome's seaport, but due to silting the site now lies 3 kilometres (2 miles) from the sea. The site is noted for the excellent preservation of its ancient buildings, magnificent frescoes and impressive mosaics.