The square of Ostia Antica, with the church of Santa Aurea

Ostia Antica is a large archeological site that was the harbour city of ancient Rome, which is approximately 30 kilometres (19 mi) northeast of the site and close to the modern town of Ostia. "Ostia" in Latin means "mouth". At the mouth of the River Tiber, Ostia was Rome's seaport, but, due to silting and a drop in sea level, the site now lies 3 kilometres (2 mi) from the sea. The site is noted for the excellent preservation of its ancient buildings, magnificent frescoes and impressive mosaics. (from wikipedia, more)