From the Super Mario Wiki, the Mario encyclopedia
The Gladiator's Spear is an item in the NES, SNES and PC releases of Mario is Missing!. It is one of the original spears used by gladiators in ancient Roman combat. It was being exhibited in the Colosseum in Rome, but it gets stolen by a Koopa Troopa under Bowser's orders. When Luigi visits Rome, he takes it back, then questions the various locals about it. A scientist, for example, dates it back to 72 AD. After gathering enough information, he takes it to the Colosseum's information booth, at which point he answers various questions to prove the authenticity of the Spear. After confirming it, Luigi returns the Spear, and the Colosseum is opened up for tourists again.