Marcopper mine, Marinduque

Marcopper Mining Corporation, 39.9% owned by Canada's Placer Development (later Placer Dome) and the rest by the Philippine state, began mining copper from the Tapian pit in 1969. Until 1975 the tailings went to an impoundment north of the pit; from 1975 they were discharged near the shore into Calancan Bay. By 1985 at least 120 million tonnes lay in the bay; estimates for 1975–1990 run to 200–300 million.

In 1991 mining moved to the San Antonio pit and, at the government's direction, tailings were placed in the old Tapian pit, after plugging the tunnel that drained it to the Makulapnit River. In December 1993 the Maguila-guila siltation dam failed during a typhoon and a debris flow went down the Mogpog River. On 24 March 1996 the tunnel plug failed: 1.5 to 3 million m³ of sulphidic slurry (UNEP estimate) poured into the Makulapnit, the Boac and the sea. The mine closed that day.

After the spill Placer Dome sold its stake but promised to clean up the rivers. Its subsidiary PDTS built berms out of the tailings themselves along the lower Boac, dredged a channel 20 metres deep and about 1 km long near the mouth to trap the slurry, and re-plugged the tunnel. USGS geologists visiting in 2000 found the pit lakes rising with a pH of about 4, tailings deposits in the Boac that release acid and copper on contact with water, and the Mogpog acidic (pH 4.5) 5 km below the mine.