Conveniently located right on US-23, the main route up the sunrise side of Michigan,Thompson’s Harbor is an undeveloped and lightly used state park that contains miles of spectacular Lake Huron shoreline. The 5,247-acre park includes 7.5 miles of Great Lake shoreline and is the largest unit on the east side of the state but is best known for having the world’s largest population of dwarf lake iris.

Despite two master plans that recommended everything from a lodge to a swimming pool, development of Thompson’s Harbor has been almost nonexistent since its dedication. For the most part the facilities at the park are limited to a trailhead parking area, two vault toilets, and a pair of rustic cabins that were added in 2009 and are available for rent. The park’s 6-mile trail system, partly composed of old two-tracks, is centered around three interconnecting loops ranging in length from 1.4 miles (Loop 2) to 2.6 miles (Loop 3) that can be easily combined for a 5.2-mile hike.

Due to the limestone character of the land, the park's terrain includes whitish marl marshes, shallow lakes, and old shorelines of cobblestone that today are traversed by the trails. Wildlife ranges from more than 100 species of birds, including nesting eagles, to deer, coyotes, and even black bears.

The limestone also makes mountain biking, disdained or prohibited in many fragile lakeshore parks, an ideal activity at Thompson's Harbor. Off-road cyclists for the most part bounce along the rocky terrain following a trail system that holds up well under knobby tires spinning along the shoreline. The ride at Thompson's Harbor may be jarring at times but the park makes for a level, non-technical ride that is extremely scenic.

The park’s best known natural feature, one that attracts a number of naturalists, are its colonies of dwarf lake iris. The smallish purple flower is a federally threatened species found only in Michigan, Wisconsin and parts of Ontario along the shores of Lake Michigan and Lake Huron. The flower is easy to spot in Thompson’s Harbor because its preferred habitat are old beach ridges of limestone, something that is abundant in the park. Arrive in June and you’ll more likely be overwhelmed by the large number of lady’s slipper orchids encountered here.