日本冲绳宫古岛耕作层下面是透水的多孔质珊瑚礁石灰岩,再向下是不透水的泥岩。该地区几乎看不到地上河流,天上降雨直接渗透到地下,在泥岩层汇集形成的地下水流,向海洋方向运动。其流量相当于降雨量的40％。为了开发地下水资源,日本在宫古岛修建了皆福试验地下坝。皆福地下坝于1977年兴建,1978年峻工。目前正在进行各方面的试验工作。 Below Miyakojima, Okinawa, Japan, there is permeable porous coral limestone below which water-impervious mudstone falls. Almost no aerial rivers can be seen in this area. Rainfall in the sky directly penetrates into the ground, pooling the formed groundwater flow in the mudstone layers and moving toward the ocean. Its flow is equivalent to 40% of rainfall. In order to develop groundwater resources, Japan built Miyako underground dam in Miyakojima. Jiefu underground dam was built in 1977, completed in 1978. Various tests are currently under way.