Wisconsin authorities say an abandoned mine where they're searching for three missing boys is a labyrinth of tunnels that goes on for miles.

Dodge County Sheriff Dale Schmidt described the Iron Ridge mine to reporters during a news conference Monday as a vast maze of interconnected tunnels that goes on for four miles. He says it would be easy to get lost in the tunnels without a compass.

The three boys — Tate Rose and Zachary Heron, both 16, and 15-year-old Samuel Lein — were last seen Sunday. Schmidt said their bikes were found off a road near an entrance to the mine.

Searchers are using dogs and thermal imaging.

The mine is owned by the University of Wisconsin-Milwaukee, which says an estimated 100,000 bats live there.