Which Roman emperor built the Colosseum's hypogeum?

The Roman emperor who ordered the Colosseum’s hypogeum built was Domitian.

The hypogeum was the underground complex beneath the arena floor. It contained tunnels, cages, lifts, trapdoors, and machinery used to move gladiators, animals, scenery, and props into the arena. It was not part of the Colosseum’s original construction.

Vespasian began the amphitheatre around AD 70–72, and his son Titus completed it in AD 80. Domitian later remodelled the building and commissioned the hypogeum, probably during his reign from AD 81 to 96. That timing explains a common mix-up: Vespasian and Titus built the main structure, but Domitian added the famous subterranean stage.

The hypogeum also changed the kinds of spectacles the Colosseum could host. Once the underground levels occupied the space beneath the arena, large-scale flooding for mock naval battles was no longer practical. The surviving tunnels remain one of the clearest reminders that Roman arena shows depended on sophisticated engineering as well as combat.