The Impact Crater is the home of the Metroid Prime (organism) which in a way resembles the mutated form of the Emperor Ing from Metroid Prime 2: Echoes.The Impact Crater is also the only place you'll find the rare Fusion Metroids, which must be destroyed with a Power Bomb. Throughout the game, you are warned about something huge down in the Impact Crater, and are cautioned to not go down there.
 Metroid Prime
Metroid Prime was the only game in which the Impact Crater appeared. It was the final area you visited. This was the only game in which the Metroid Prime had appeared.