Kawarage Jigoku, located in the post-caldera eruption center of the Pliocene Sanzugawa caldera, is known for its vigorous fumarolic activity and hydrothermal silicification. In this active geothermal field, the most intensely silicified area 500 m long and 300 m across includes circular or horseshoe-shaped four depressions 30-50 m deep and 120-160 m across, in which fumaroles, including those that are no longer active, are formed mainly along the inner walls. These depressions have been excavated to mine the natural sulfur deposited around the fumaroles over the last 500 years, and local people have never been concerned about the origin of the depressions. However, we recently found that pyroclastic density current (PDC) deposits cover the rims and outer slopes of the depressions with a thickness of centimeters to meters and are composed of silicified rock fragments ranging in size from lapilli to ash grains. This finding suggests that the depressions are craters formed by phreatic eruptions. The PDC deposits characteristically contain amoeba-like vesicles that are traces of vapor trapped from the PDCs. In addition, some constituent fragments have jigsaw cracks filled with quartz, providing evidence for hydraulic fracturing of the host rock due to sudden boiling of hot water at depths.