From the Smithsonian magazine:
The floor of the colosseum, where you might expect to see a smooth ellipse of sand, is instead a bewildering array of masonry walls shaped in concentric rings, whorls and chambers.
Heinz-Jürgen Beste of the German Archaeological Institute in Rome, has spent much of the past 14 years deciphering the hypogeum — from the Greek word for “underground” - the extraordinary, long-neglected ruins beneath the Colosseum floor.
“The hypogeum allowed the organizers of the games to create surprises and build suspense,” Beste says. “A hunter in the arena wouldn’t know where the next lion would appear, or whether two or three lions might emerge instead of just one.” This uncertainty could be exploited for comic effect. Emperor Gallienus punished a merchant who had swindled the empress, selling her glass jewels instead of authentic ones, by setting him in the arena to face a ferocious lion. When the cage opened, however, a chicken walked out, to the delight of the crowd. Gallienus then told the herald to proclaim: “He practiced deceit and then had it practiced on him.” The emperor let the jeweler go home.
Secrets of the Colosseum. Smithsonian magazine. 01/2011.