The Aitken crater on our moon is the largest impact crater in the solar system.

This crater is 2500 kilometers in diameter and over 12 km deep; a truly vast feature of the moon, when you consider its size.

The presence of this crater was confirmed during the Lunar Orbiter program in the mid-1960s, but the photographs did not do justice to the size of the crater. In appearance it only looks like a dark smudge in the grey face of the moon, and initially it was taken to be due to the lunar rocks in this region containing more iron bearing minerals than the surroundings.

It wasn't until the Clementine mission that the anomaly was seen to be definitely a crater. This craft was equipped with a laser altimeter and a camera able to photograph over 11 different wavelengths, showing the dark area was indeed composed of different minerals than the surroundings, and over 12 km deep.

This crater is also interesting, as the theory of crater formation says that the floor of the crater should be composed (almost) entirely of mantle, from below the moon's crust. However the spectroscopic data seems to indicate that, at best, only 50% of the material is mantle. Also, why was the moon not shattered by such a huge impact?

A favoured hypothesis is that the projectile that caused the crater struck at a low angle; less than 30 degrees,which lessens the energy available to dig the crater.

A lot of work is still going into this crater though, and the final answer may well be different from any of the theories put forward so far.

Summarised from the web page :-
Written by G. Jeffrey Taylor Hawai'i Institute of Geophysics and Planetology