The first clue to an abandoned mine is a seemingly random pile of rocks, left over from what was carted out of the ground to create a tunnel.

These dump piles generally are near a mine opening, or portal, which is referred to as an adit if it's horizontal and a shaft if it's vertical.

"The portals are the most dangerous part of an adit because the surface rocks are the most fractured," Joan Gabelman, a geologist with the Bureau of Land Management, said as she peered into a deep, dark hole with a grate over it during a recent tour through the Scratchgravel Hills.

The horizontal tunnels into the mine are called drifts, and the nearly vertical tunnels below the drifts are known as winzes, which typically are steeply inclined to connect one mine level with a lower level. Some mines also have "stopes" -- a mining term for chambers in which ore is broken and mined, usually from the bottom up. These stopes often weren't filled when the miner left a claim and can collapse upon themselves as the earth shifts, sometimes breaking through to the surface, in a process called stoping.

These shafts, adits, winzes, stopes and drifts are dangerous in a number of ways.

Sometimes, the lower drifts can fill with water and if a person slides down a winze, it's difficult to get back up.

Or a miner leaving a claim might put a few boards over the winzes or the mine shaft opening, unknowingly creating camouflaged hazards.

"After 100 years, the wood rots away and you jump on it, and you're gone," said Vic Andersen, Montana's Mine Waste Cleanup Bureau chief.

Gabelman added that there might not be any waste rocks by the shaft, so people can't count on that to detect abandoned mines.

Items left within the mines -- like dynamite, which has been found by the caseload in some mines -- pose another hazard. Gabelman recalls one recent case in which the sticks were crystallized to the point that the BLM blew them up in the mine rather than try to move them.

Even more of a hazard are blasting caps, which can be scattered by rodents and will explode if stepped on.

If that's not enough, Gabelman adds that many mine ceilings were made by wedging timbers across the top. As the earth shifts and the wood rots, just a bit of jarring can collapse the roof.

Still, the mysterious openings into hillsides are a lure for children and others.

"When we go into adits often we find signs that some kids -- kind of like Tom Sawyer -- were in there and maybe tried to build fires near the portal," Gabelman said. "One of the dangers of being in a mine is the air quality, and a fire uses oxygen, so you could hurt or kill people by building a fire by the portal."

Back in the 1980s and 1990s, the state basically shoved culverts into the portals and covered the opening with plywood or grates, but those were so easy to break into that many were replaced with sturdier metal and sometimes stuffed with an insulation type material known as PUF, or polyurethane foam.

Andersen and Gabelman note that before any work is done they have to figure out whether any sensitive plants or species are in the area -- the "Townsend's big-eared bat" is know to frequent mines here -- and the state and federal agencies also must do historical evaluations. The closed portals often include horizontal slots that allow bats access.

"We had our archaeologist draw a map of what was here, and we took a lot of photographs, then submitted it to the state historic preservation office," Gabelman said at one of the mines they closed. "We don't want to erase history, because what do you have left? The idea is to make it safe."

Reporter Eve Byron: 447-4076 or eve.byron@helenair.com