It is possible that the Yukatan Peninsula is the remains of a very large impact crater.
Impact craters form when lumps of rock floating in space collide with the solid planets.
Until about 4 billion years ago there was a lot of debris in the Solar System left over from when the planets were formed. This debris gradually hit the planets until there was none left. Every time a bit of rock hit the Earth, for example, an impact crater was formed. This is similar to the splash formed when a pebble is thrown into a pond. In this case though, because the rock is solid, the 'splash' remains leaving a gapping hole in the ground.