Recently installed seismic detectors in Yellowstone National Park have been detecting unusual patterns, according to a study by USGS scientists covered in the Billings Gazette. According to scientists, the detectors are actually measuring waves on Yellowstone Lake.
Wind can push water across a lake, pushing up the water level on one side of a lake relative to the other. This can lead to seiche waves that sway back and forth. The long wavelengths of these waves make them almost unperceivable to people, but the sheer volume of displaced water has been detected by local seismic detectors as the weight of the water literally moves the ground around it.
Given the high density of water, the weight of water moved in Yellowstone Lake has been estimated to be around 5 million tons, exceeding that of the entire U.S. Naval fleet.