What had created those strange round “craters?" (“Krater" is Greek for a bowl or wide-mouthed goblet.) They reminded some observers of volcanic craters on Earth, or better, of the large “calderas" (cauldrons) formed by the internal collapse of volcanos, e.g. Crater Lake (http://www.crater.lake.national-park.com/) in Oregon. Others suggested that they were formed by the impact of large meteorites, but this was countered by the argument that most meteorites probably arrived at a slanting angle, and were expected to leave not a round ring but an elongated gouge.
We now know that the impact explanation was right. The craters are round because at the enormous velocities with which meteorites arrive, the impact resembles a local explosion, and the signature of the impact is determined by the energy released rather than by the momentum transmitted.
Part of the evidence has come from the nicely rounded impact remnants found on Earth, e.g. Meteor Crater (Canyon Diablo) in Arizona and Manicouagan lake in Canada, in northern Quebec (see picture), which is about 100 km (60 miles) wide and 214 million years old. Note that rather than having a pit in its center, the Manicougan lake has a round island. After the impact, the land rose again to the level of its surroundings, pushed by fluid pressure of the material below it, which acts like a viscous fluid and tries to establish equilibrium between the different loads which it supports. (For another picture of Lake Manicouagan, and more about it, click here: http://antwrp.gsfc.nasa.gov/apod/ap001213.html)
Other solid bodies of the solar system also display round craters. On the large ice-covered moons of Jupiter, the return to equilibrium is much more pronounced, because ice sags and flows much more readily than rock. Those moons display “palimpsest" (named so after scrolls of parchment that were ‘recycled’ by having their original text scratched off) craters which are merely surface markings, because as time passed, the walls which originally existed sagged back onto the flat surface.