Mare Tranquillitatis will live forever in the annals of exploration, thanks to Apollo 11, which made the first manned lunar landing on its southwestern surface in July 1969. However, to most backyard lunar observers, Tranquillitatis is one of the duller maria to view, with less topography than others. Tranquillity Base, where Apollo 11's astronauts landed, lies on the southern part of the mare, near the impact crater Moltke (4 miles wide), which is surrounded by a white halo of ejecta. No easily visible landmark stands close to Tranquillity Base, but if seeing allows, look for the trio of small (1- to 2-mile-diameter) craters named Armstrong, Collins, and Aldrin. Rocks brought back by the Apollo 11 astronauts sampled two lava flows, one dated at 3.84 billion years old and the other 3.57 billion. The different ages show the volcanic process that filled the Tranquillitatis basin took at least 270 million years. The sculpture of the hills around Dionysius comes from the Imbrium basin to the northwest. Debris shooting from the Imbrium impact site on low trajectories scoured the terrain deeply. The 2- to 3-mile-wide Rima Ariadaeus, however, cuts across the Imbrian sculpture at an angle. If the rille was produced by a basin impact, as geologists think likely, then the yet unidentified basin lies near the center of Oceanus Procellarum, which was filled by lava many eons ago. The ancient crater Julius Caesar (56 miles) has been heavily reshaped by Imbrian ejecta (the lighter material in the crater's southern half) and by mare flooding. East of Julius Caesar, the crater Sosigenes (11 miles) lent its name to a set of rilles resembling off-road tire tracks. They were caused when the mare lava sheets settled and fractured.