Arènes de Lutèce, the ruins of the 1st century AD Roman amphitheater where gladiators once battled, is hiding in plain view, surrounded by apartment buildings. The easiest of three entrances to find is at 47, rue Monge, which looks as though it leads into a courtyard. When the ruins were uncovered in the mid-19th century, Victor Hugo campaigned to preserve them. Today the space is a public park.