Yet, despite unsuitable ground and weather conditions at the site in northern Sumatra, Indonesian officials approved an application for a tailings dam in 2005 that experts say was already flawed.
They are now debating whether to approve an updated proposal to extend the dam — which has not yet been built — beyond its original scope.
"A failure of the tailings dam would lead to instant death for thousands of people with no warning, no time for evacuation," said Steven Emerman, an associate professor of Hydrology at Utah Valley University in the US, who has studied the dam on behalf of local communities.
"Considering that the consequences would be severe and immediate, any tailings dam should meet the absolute highest safety standards possible — and we find the exact opposite here."
Unstable tailings dams
Unlike dams to hold back water, tailings dams store waste material dug out of a mine such as rocks and minerals that are sometimes toxic.
They are often built cheaply and with little regulation, which has contributed to them failing at a rate about 100 times greater than that of water dams, according to a study published in the journal Geotechnical News in 2010.
The dam near the village of Sopokomil will use a rock wall, which is sturdier, but experts say the design is vulnerable to liquefaction that could cause it to tip when earthquakes and heavy rain turn wet ash into sludge.