Only the rim of the Earhart crater is still visible. The rest of it was obliterated by another impact that formed the massive Mare Serenitatis (Sea of Serenity) 3.9 billion years ago. The researchers were only able to figure out it existed based on data from NASA's GRAIL mission which mapped the Moon's gravity field. When the scientists looked at the resulting gravity maps, they noticed a circular crater-like anomaly whose circumference lined up directly with a semi-circle shaped ridge (part of the rim of the crater). The researchers decided that the elusive crater should be named after Earhart, a proposal they hope will be approved by the International Astronomical Union.