UPDATE: So, about that 'young' asteroid impact crater in Greenland... it's actually old. Really old.

Some time in the past, an asteroid a kilometer across slammed down into Greenland at a speed at least a dozen times faster than a rifle bullet. Releasing as much energy as 15,000 times the largest nuclear weapon ever exploded on Earth, it carved out a gigantic crater now called Hiawatha Crater, and it's over 30 kilometers across.

This much is known. The big question is, when did this happen?