The Romans connected the river with a sewer system (the "Cloaca Maxima") and with an underground network of tunnels and other channels, to bring its water into the middle of the city. Wealthy Romans had garden-parks or "horti" on the banks of the river in Rome up through the first century BC. These may have been sold and developed about a century later. The heavy sedimentation of the river made it difficult to maintain Ostia, prompting the emperors Claudius and Trajan to establish a new port on the Fiumicino in the 1st century AD. They built a new road, the "via Portuensis," to connect Rome with Fiumicino, leaving the city by Porta Portese ('the port gate'). Both ports were eventually abandoned due to silting. Several popes attempted to improve navigation on the Tiber in the 17th and 18th century, with extensive dredging continuing into the 19th century. Trade was boosted for a while but by the 20th century silting had resulted in the river only being navigable as far as Rome itself. The Tiber was once known for its floods — the Campus Martius is a flood plain and would regularly flood to a depth of . The river is now confined between high stone embankments which were begun in 1876. Within the city, the riverbanks are lined by boulevards known as "lungoteveri", streets "along the Tiber". Because the river is identified with Rome, the terms "swimming the Tiber" or "crossing the Tiber" have come to be the Protestant shorthand term for converting to Roman Catholicism. This is most common if the person who converts had been Anglican, the reverse of which is referred to as "swimming the Thames" or "crossing the Thames". In ancient Rome, executed criminals were thrown into the Tiber. People executed at the Gemonian stairs were thrown in the Tiber during the later part of the reign of the emperor Tiberius. This practice continued over the centuries. For example, the corpse of Pope Formosus was thrown into the Tiber after the infamous Cadaver Synod held in 897. In addition to the numerous modern bridges over the Tiber in Rome, there remain a few ancient bridges (now mostly pedestrian-only) that have survived in part (e.g., the Ponte Milvio and the Ponte Sant'Angelo) or in whole (Fabricius' Bridge).