SAND LAKE, Mich. (AP) -- About 15 million gallons of partially treated sewage water disappeared from a 250,000 square-foot storage lagoon into a sinkhole, but officials don't know where it went after that.

Kent County utility operator Nathan Danenberg, who runs the sewage treatment system for Sand Lake, discovered the leak in the 8-foot-deep lagoon on Friday while taking samples. It wasn't clear when or why the leak occurred.

"I don't know if maybe there are old mines in the area," Danenberg told The Grand Rapids Press for a story published Tuesday.

"It's an odd case. A sinkhole gobbled up all the water and we don't know where it went... It seems to have just gone down into the earth.

"We don't smell anything and we don't see anything."

Mike Bolf, drinking water district engineer for the Michigan Department of Environmental Quality, said the state would conduct tests on nearby municipal wells, which are downstream as groundwater flows. But the wells are believed deep enough to be protected by a natural clay or shale wall, he said.

The lagoon, lined with clay, is one of three that house sewage from the village during winter while it is treated with bacteria.