The Colosseum is the biggest amphitheatre ever built. An amphitheatre is a round, open-air stadium with seats all the way around, so everyone can see the middle. The Romans used it for chariot races, athletic shows, plays, music and even pretend sea battles where they flooded the floor.
From the outside it has four storeys of stone arches stacked on top of each other - 80 arches on each level on the bottom three rings. The Romans invented this design so a huge crowd could come in and out quickly through different doors. A full Colosseum could empty in about 15 minutes.
Underneath the main floor was a hidden network of tunnels and rooms called the hypogeum. Wooden lifts pulled by ropes brought scenery up onto the stage, like a giant magic trick. There were 80 of these little lifts. Scientists and engineers are still working out exactly how they all worked together.
The building has lost some of its stone over the centuries - earthquakes shook bits off, and other people borrowed the stone to build new things. But the part that is left still has its huge arches, and millions of visitors walk inside it every year to imagine the cheering crowds.