Thunder Bay — Alpena

A day here is run out of Alpena's harbour on a small charter, and the lake decides how far it goes: the outer wrecks need a flat morning, while the inshore hulls stay workable when it is blowing. The sanctuary sets seasonal mooring buoys on a number of the wrecks, so boats tie in rather than hook a hull, and crews plan two dives with a long surface interval to keep the cold in check. Nothing about the rhythm is tropical. You dress before you leave the dock and peel off again between dives.

There is no reef and no wall; the lake floor is sand, clay and limestone, and the ships are the only structure on it. That splits the diving by depth more than by site. Above the reach of storm waves and winter ice the hulls are broken open and spread across the bottom, which makes them easy to swim and easy to read. Below that line the weather never got at them, so they sit closer to the way they went down, and the deeper dives feel less like wreckage than like a ship parked in the dark.

The sanctuary was designated in 2000 and enlarged in 2014, and everything inside it is protected, so artefacts stay where they lie and the interest is in looking rather than handling. Most visitors start at the Great Lakes Maritime Heritage Center in Alpena, the sanctuary's visitor centre, which explains what they are about to swim over. Fresh water is worth a thought as well: anyone who weights themselves for the sea will turn up carrying more lead than the job needs.

This suits people who come for history rather than animals, and who accept diving colder than a warm-water background prepares them for. A drysuit is the sensible default even in August, and cold-water skills matter more here than depth experience. Non-divers are not an afterthought either, since glass-bottom tour boats run over the shallow hulls out of Alpena and some of the same ships can be paddled over. Anyone after warm water, big fish or three tanks before lunch should look somewhere else entirely.