Early metallic mineral exploration in Michigan's upper peninsula resulted in the creation of thousands of vertical and horizontal mines and pits. These sites, after mining operations ceased, became depositories for trash, hazardous places open to curious explorers and shelters for bats and other cave dwellers.

Mines offered optimum bat winter roosting sites (called hibernaculum) due to their relatively constant temperatures, around 54 degrees, high humidity and low air movement.

One example of the importance of these old mines is the Millie Hill site in Iron Mountain, Michigan. Conservation practices were deemed necessary after a survey into the abandoned mine by Marquette fireman and part time spelunker Steve Smith discovered a population estimated at one million little brown and big brown bats. The mine was heavily vandalized and slated to be closed, threatening the unique resource.

A partnership developed between the Natural Heritage Program, the local community, Bat Conservation International and other state and federal agencies to develop a management plan, including a cage designed to keep humans from entering the shaft while allowing the continued use of the site by bats. Educational programs were conducted to stress the importance of bats in the environment.

Thanks to the concern and involvement of all the individuals and organizations. The initial phase of the plan seems to have worked. Bats in the Millie Hill mine are protected and have free movement in and out of this abandoned iron mine through the "bat cage".

This mine has been designated as having significant natural resource features and is now a Watchable Wildlife viewing area. Improvements are being developed to allow safe public visitation to the site.