A jogger found the huge hole on Issaquah's Squak Mountain Tuesday.

"That's what I felt instantly when I saw it, that's a massive hole in the earth!" exclaimed Gillian Middlestead, the jogger who found the hole

"I was running down the sidewalk and saw basically a huge hole in the earth," she said. "I leaned over and couldn't see the bottom. I could also see that it was a cavern, and even where I was standing that there was nothing underneath me."

The bottomless pit turned out to be an old air tunnel that connects to twin mine shafts underground. The old shafts were capped off decades ago.

"I'm standing on one of them right now that went out to the bottom of the creek," explained Todd Jensen with Issaquah's Public Works team. "The other one is sitting right over there, on the other side of the hole, is another shaft."

Crews were working around the clock Wednesday to fill in shaft with rocks and concrete before anyone got hurt.

"Just 30 minutes before 15 school kids had stood across the street waiting for the bus," said Middlestead.

Crews say the entire hillside on SW Gibson Lane is a virtual honeycomb of old mining shafts and tunnels.

"The whole hillside up there was developed for the mine," explained local historian Linda Hjelm.

She says the Gillman Coal Mine started in 1888. Coal was mined on Squak Mountain for more than 50 years. The mineshafts were capped over the years but they periodically re-appear.

The last cave-in happened in the same area back in 1989.

"There's an awful lot of area up there where there were various entrances and they're real close to the surface," said Hjelm.

It's an uncomfortable prospect for families living on the old mine shafts -- some who didn't even realize they were there.

"After the Tsunami's you realize how incredibly fragile we are and the power of the earth," reflected Middlestead. "We appear to have engineered ourselves to safety but that really isn't the case."

A giant hole in the middle of a neighborhood sidewalk certainly was testimony to that.