Fontana Dei Fiumi or Fountain of the Four Rivers

Obelisk 3, originally uploaded by mharrsch.

"In the centre of the Piazza Navona is Bernini's most spectacular fountain, la Fontana dei Fiumi, erected in 1651. It features a central rocky structure that supports an obelisk that was an ancient Roman imitation of the Egyptian form. Around this structure are four giant statues by Bernini's pupils following his designs representing the Nile, the Danube, the Ganges, and the Rio della Plata, each representing one of the four quarters of the world." - Roma Interactive