Old Issaquah mine entrance, once sealed, is slowly reopening

The hole was about the size of a small book when Steve Grate spotted it in January.

Now the old Issaquah Grand Ridge Mine entrance has widened to 1-½ feet in diameter, the size of an office-chair seat.

The location of the hole has officials worried. It sits in a 15-foot-wide depression next to a popular hiking and mountain bike trail south of Grand Ridge Park in Issaquah Highlands.

Like other old mine entrances and shafts that open occasionally on Cougar and Squak mountains, this small piece of Eastside history could swallow a stray animal or unwary hiker.

The growing size indicates the debris used to seal the mine shaft 50 years ago has deteriorated. Anyone stepping into the depression to get a closer look at the hole could fall through the thin crust of dirt overlaying the mine opening.

"The key is to stay on the trails," said Grate, a computer consultant who's also a local mine expert and hiker.

Wednesday, Grate led a team of experts — including King County Parks and Recreation rangers, a historian from the Issaquah Historical Society, a geotechnical engineer and a representative from the Department of the Interior — to inspect the site. He brought maps and historical photographs of the Grand Ridge Mine.

Grate, fascinated with the connecting parks on Cougar, Squak and Tiger mountains, delved into the history of the area. After more than three years of research, the Issaquah resident has compiled the area's largest collection of maps, photographs and GPS coordinates of more than 50 mines that operated on the Eastside from 1863 to 1963. He leads hikes through the old mining areas for the Issaquah Historical Society and is writing a book about what lies beneath the trails and housing developments.

Miners took coal from the Grand Ridge Mine from 1909 to 1934 and then sporadically through the 1950s. It was the last mine to close in Issaquah.

When Grate alerted federal officials to his discovery of the mine entrance, Ginger Kaldenbach instantly understood the problem. She supervises inactive mines throughout the western United States for the Office of Surface Mining in the Interior Department.

"When they were done mining, they used to dump old stumps, car bodies, slop that they didn't want into the entry and cover it up," Kaldenbach said. "That only lasts 30 to 50 years."

Since 1977, the Department of the Interior has worked to clean up hazards from closed mines. Emergencies such as the Grand Ridge hole, which could threaten human life, take top priority, Kaldenbach said. Workers will excavate the area down to the rock around the mine entrance, then plug it with a concrete barrier. The work will take several months to complete.

Bobbi Wallace and David Kimmett, both of King County parks, were more concerned about the immediate safety of hikers. They will oversee fencing of the area in the next few days.

Wallace also worries about mountain bikers and geocachers — people who take part in scavenger hunts using handheld Global Positioning System (GPS) units.

In their eagerness, they often go through the underbrush instead of staying on designated trails, she said.

"In a few weeks, the nettle and underbrush will have grown so much people won't see something like a hole or a depression," Wallace said.

Hiking with Grate was an educational experience for the team.

What looked like a small hill was a pile of discarded trailings from the mining operation. He pointed out cement foundations and footings from the mine's buildings.

"I always thought those were leftover from the construction of I-90," said Kimmett, a natural-resource coordinator.

Grate showed the group two other mine entrances, both still sealed, and photographs of the mining operation.

He picked up pieces of coal lying atop the trail and handed them around.

Grate describes the old mines as "deathtraps." Many went under the water table, so old tunnels may be flooded.

The shafts that pumped fresh air into the mines were also filled in, so poisonous methane gas permeates many mines.

Plus, the 100-year-old timbers holding up walls and roofs have deteriorated.

"There isn't an emergency crew in the county that can rescue you if you fall into an old mine," Grate said.

Sherry Grindeland: 206-515-5633 or sgrindeland@seattletimes.com