Divers have their own sanctuary just off the Alpena shore. Here, the Thunder Bay National Marine Sanctuary and Underwater Preserve (www.thunderbay.noaa.gov) thrills divers with its clear waters, interesting underwater limestone formations, and shipwrecks.

Rocky islands and hazardous shoals proved treacherous for mariners; the preserve protects some 80 shipwrecks, 14 of which can be explored with the help of a wreck-diving charter. Among the most popular are the Nordmeer, a German steel steamer that sank in 1966, and the Montana, a 235-foot steamer that burned and sank in 1914.

Thunder Bay Scuba (413 S. Ripley Blvd., Alpena, 989/356-6228, www.tbscuba.com, 9 a.m.–5:30 p.m. Mon.–Fri., 10 a.m.–2 p.m. Sat.–Sun.) offers charter diving services for about $44 per day, departing from Alpena’s city marina.

The state established the Thunder Bay National Marine Sanctuary and Underwater Preserve—more than 288 protected miles in all—in 1981, largely to prohibit divers from removing artifacts from the site. In October 2000, the state preserve was also designated a national marine sanctuary, a status that grants it federal funding and additional resources for scientific and archaeological study.

For more information, contact the Michigan Underwater Preserve Council (800/970-8717, www.michiganpreserves.org).