On Mars, the Hellas Basin is a large impact crater.  It has a depth of 7 km plus it is about 2000 km across.  It could also fit half of the United States which is east of the Mississippi.  It is close to being three times as wide as Argyre (another huge crater on Mars) and that is really big.  Some scientists believe that three billion years ago the crash of a big asteroid formed what is now Hellas Basin.  These craters can tell us something like water could have been in those craters millions and millions of years ago.