Lava tubes and related flow structures were first recognized upon examination of Viking orbiter images, and later identified using orbiter imagery from Mars Odyssey, Mars Global Surveyor, Mars Express, and Mars Reconnaissance Orbiter. Lava tubes can visually be detected two ways. The first is as long sinuous troughs known as rilles, which are believed to be the remains of collapsed lava tubes. The second method of possible identification is through observation of cave skylights or pit craters, which appear as dark, nearly circular features on the surface of Mars. In June, 2010, a group of seventh grade science students at Evergreen Middle School in Cottonwood, California, participating in the Mars Student Imaging Project, helped researchers discover a new series of lava tubes near Pavonis Mons through identification of a skylight estimated to be 190×160 meters wide and at least 115 meters deep. It is only the second skylight known to be associated with this volcano. In addition to orbital imagery, lava tubes could be detected through the use of: