Special about Lowell crater is a ring of mountains 90 kilometers in diameter in the crater center. Similar peak-ring impact craters have been observed also on Earth, Venus, Mercury, and the Moon. In general, central peaks and peak-rings form by gravity-driven collapse and uplift of the floor following crater excavation. The collapse of craters of a certain diameter can give rise to a complex interior structure with flat floors, central peaks, wall terraces, and internal rings, whereas smaller impact craters have a simple bowl shape. The transition diameter of so-called simple to complex craters depends on the gravity of the planet, as well as on the target material strength. When a complex crater reaches a diameter of more than ~45 kilometers (Mars), or ~25 kilometers (Earth), the central uplift can create a ring of irregular mountain peaks in the crater center.