What is this? (これ、な～んだ？, Kore Na~nda?, lit. This, Wha~at?) is an event in Tomba!.
Find out about the strange thing you found in the Watch Tower.
In the Watch Tower, go to the second wooden platform and use Tomba's equipped weapon to break the right side of it to reveal a hidden object and activate the event. The object hasn't a purpose at the moment and will be used later in the game.
After you find the way to the Underground Maze the object that is in the Watch Tower will be unlocked, then you have to go there, get close and the event will be cleared.