Map showing the location of the Mizunami Underground Research Laboratory (MIU) and the Ontake Kougen Hotel (OKH), where continuous gravity observations were performed. (a) Regional map of Gifu, Nagao, and Aichi prefectures, Central Japan. The GPS station TOKI (ID:020994 of GEONET of the Geospatial Information Authority of Japan) is also shown. (b) Topographic map with roads around the MIU construction site. The closed square denotes the refuge area of the 100-m sub-stage (at a depth of 100 m below the surface) where a gPhone-90 was set up during the summer of 2010. A sub-stage is a horizontal tunnel between the Main shaft (northeast open circle) and the Ventilation shaft (southwest open circle) at every 100 m in depth. The label MGA denotes the Mizunami Geoscience Academy building. (c) Topographic relief map of the eastern flank of the Mt. Ontake Volcano. The gPhone-90 was set up in the elevator engine room of the OKH (solid square) from the winter of 2010 to the spring of 2011. The two open squares show the Automated Meteorological Data Acquisition System (AMeDAS) stations (KID, Kaida-kogen; ONT, Ontakesan) of the Japan Meteorological Agency (JMA).