The enormous arena was empty, save for the seesaws and the dozens of condemned criminals who sat naked upon them, hands tied behind their backs. Unfamiliar with the recently invented contraptions known as petaurua, the men tested the seesaws uneasily. One criminal would push off the ground and suddenly find himself 15 feet in the air while his partner on the other side of the seesaw descended swiftly to the ground. How strange. In the stands, tens of thousands of Roman citizens waited with half-bored curiosity to see what would happen next and whether it would be interesting enough to...