Kathy Sullivan, the astronaut who in 1984 became the first American woman to walk in space, has now become the first woman to reach Challenger Deep, the lowest known point on the planet. Sullivan, 68, was aboard a submarine that dove 6.8 miles below the sea’s surface in the Mariana Trench on Sunday night. She and the submersible’s chief pilot, Victor Vescovo, 54, a private-equity investor who became the first person to hit the deepest point of five oceans last year, celebrated their return by making a call to the International Space Station. “As a hybrid oceanographer and astronaut this was a once in a lifetime day—seeing the moonscape of the Challenger Deep and then comparing notes with my colleagues on the ISS,” she said last night. Sullivan is only the eighth person to visit Challenger Deep, which is within the Mariana Trench in the western Pacific, and was the head of NOAA after her space career ended. It was first reached by Jacques Piccard, the Swiss oceanographer, and Don Walsh, a U.S. navy lieutenant, in 1960. In 2012, the film director and underwater explorer James Cameron visited aboard the submersible Deepsea Challenger. The trench is about a mile deeper than Mount Everest is tall, with pressure of about 16,000 pounds per square inch.