"Tethys is big, about 1100 km (660 miles) across (about 1/3 the diameter as our own Moon). Its density is actually a bit less than that of water, so it’s most likely predominantly composed of water ice. The surface is bombarded with craters, including the big one at the bottom called Melanthius. It’s 250 km across (150 miles!) and sports a massive central peak, common in larger craters. The crater itself is from a gigantic impact on the moon, and the central mountain forms when material is first displaced by the impact, then flows back. Under those titanic stresses, solid material can actually flow as the impact shock wave passes through, so these peaks are seen on lots of big objects in the solar system." - Steven Perez from Bookmarklet