This large chasm called the Sink marks the point where the roof of the cave system stopped collapsing. When Grand Gulf is empty, the bottom of the Sink and a cave opening are visible. That cave opens into a tunnel and functions like a drain, so all water that fills Grand Gulf leaves through the hole in the bottom of the Sink. After draining from the Sink, the water travels nine miles underground and resurfaces at Mammoth Spring in Arkansas. Scientists determined this underground flow with dye trace tests, which place fluorescent dye into the water and use charcoal-filled packets to test for captured dye. In 1968, a student from West Plains High School conducted the first dye trace test here as a science fair project and recovered the dye at Mammoth Spring. In 2013, the Department of Natural Resources' division of Missouri Geological Survey (MGS) carried out a detailed dye trace test that confirmed the 1968 finding.