Bellyflop

From Freepedia

The bellyflop is an undesired way of entering the water in diving (not scuba). In it, the body enters the water horizontally or nearly so, belly down. For many people, this can be rather painful. If entering water from a height, it is best to do so feet first unless trained in platform or springboard diving, or competitive swimming.

Sometimes something is said to bellyflop, meaning that it failed.