The U.S. military’s experimental, unmanned X-37B space plane landed safely on Sunday at NASA’s Kennedy Space Center in Florida, after completing a classified mission that lasted nearly two years. Built by Boeing, it blasted off in May 2015 from nearby Cape Canaveral Air Force Station aboard an Atlas 5 rocket built by United Launch Alliance. The X-37B conducted unspecified experiments for 718 days while in orbit, making this the fourth and lengthiest mission so far for the secretive program, managed by the Air Force Rapid Capabilities Office. According to the Air Force, it’s used to “perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies.” Click here to view the first image in today’s viral picture gallery. Continue reading for the five most popular viral videos today, including one showing how to escape quicksand.