National Geographic - Roman Colosseum

When Emperor Titus inaugurated the Colosseum in A.D. 80, up to 50,000 spectators crowded the four-tiered, elliptical amphitheater to watch gladiator contests and combats between men and wild animals. Though the games were discontinued centuries ago and not much remains of the stone-and-concrete structure—it has suffered a series of lightning-caused fires and earthquakes—the Colosseum is still frequented today. Tourists flock to the arch-filled, 617-by-512-foot marvel that long served as a model for stadiums worldwide.