Fontana di Trevi (Trevi Fountain). This was designed in 1732, completed in 1762, and depicts Neptune's chariot being led by Tritons with seahorses and yada yada yada.. The custom is to hurl a coin into the fountain over your shoulder while facing away and having your picture taken. This ensures your future return to Rome. A second coin grants a wish, but I was too cheap for that. In fact I wanted to lean in and get my coin back hehe.