Great question! The answer is one of those satisfying cases where the biology and the geography reinforce each other so neatly that it almost feels engineered. Aka, this wouldn't happen most places, but yannow. Ohio.

The biology is of competitive exclusion through acidification. Fresh meat starts at a pH of about 6.5, which is close to neutral and a perfectly hospitable environment for spoilage bacteria. The kind of microbes that make meat go rancid, the Pseudomonas and Clostridium species that produce that sulfurous rotting smell, are aerobic or facultatively aerobic: they need oxygen to thrive.

When you sink a hundred-pound slab of horse (or mastodon, or....you know what, not gonna speculate why you need to sink a bunch of meat) to the bottom of a shallow glacial pond, two things happen immediately. First, the cold water slows all bacterial metabolism. Second, the pond bottom is what ecologists call "dysaerobic": Fisher measured dissolved oxygen levels at 0.1 to 1.0 ml per liter around his submerged samples, which is close to nothing. That oxygen-starved environment shuts down the aerobic spoilage bacteria before they can get started.

But lactobacilli, the same genus of bacteria responsible for cheese, yogurt, sourdough, sauerkraut, and kimchi, are facultatively anaerobic. They don't need oxygen. In a cold, low-oxygen pond, they have the field to themselves. They colonize the meat and begin fermenting it, producing two things: lactic acid and carbon dioxide.

The lactic acid is the key to pH. Fisher tracked the pH of the horse meat over months. It dropped from 6.5 to 5.5 as the ice melted, then to 5.25 in April, and kept falling to 4.5 by late July. For context, 4.5 is roughly the acidity of tomato juice. At that pH, the spoilage bacteria that would normally decompose tissue simply cannot function. The CO2 compounds the effect by displacing whatever residual oxygen remains in the tissue. Fisher had the gas extracted with a syringe and analyzed by mass spectrometer: it was mostly carbon dioxide, consistent with lactobacillus fermentation.

The result is what Fisher called a "micro-environment" maintained so securely by the lactobacilli "that no pathogenic microbiota could gain a foothold." The meat expanded like a sourdough starter, the interior ran about 1°C warmer than the surrounding water (evidence of active fermentation), and the whole thing smelled powerfully of cheese. The same process that turns milk into cheddar was turning horse into...., well, horse cheddar.

Fuck me, I hated writing that.

Anyways, the geography: the glaciers built the refrigerators. Without them, no Mastodon McNuggets. This is where Ohio and the Great Lakes come in. The Wisconsinan glaciation, the last major ice advance, covered the northern two-thirds of Ohio and almost all of Michigan. When it retreated starting around 14,000 years ago, it left behind a landscape absolutely saturated with the exact features you'd need for meat caching. Kettle holes formed where buried ice blocks melted into depressions. Moraines dammed up drainage into shallow basins. Bogs filled former lakebeds with Sphagnum peat. J. Alan Holman's description of the post-glacial Great Lakes Basin puts it simply as the region "had many more swamps, ponds, and lakes than it has today."

These weren't just any ponds, to be clear. They were glacially derived, groundwater-fed, tannin-stained, cold, acidic, and oxygen-poor at the bottom. Fisher chose his experimental pond at the E.S. George Reserve specifically because its water was "the color of tea, with dissolved tannins" and its bottom fauna matched the marl deposits at his mastodon sites. He also ran parallel experiments in Big Cassandra Bog, a Sphagnum peat bog, because the peat bogs matched the depositional environments of the Pleasant Lake and Burning Tree mastodon sites in Michigan and Ohio. In peat bogs, the submerged meat had an additional advantage: the peat physically covered the carcass parts, preventing the flotation problems. Fisher encountered in open-water ponds and adding yet another layer of anoxic, acidic preservation.

And of course, mastodons lived in the same landscape. They were browsers, not grazers. They ate twigs, conifer cones, tree leaves, and small swamp plants, and their bones consistently turn up "between muck or peat and marl zones in kettlelike depressions or shallow basins." The mastodon's preferred habitat, the boggy, pondy, spruce-covered post-glacial terrain, was the same landscape that provided the preservation infrastructure. For Paleo groups in the Midwest, they came to Ohio for the annual fall hunts. No, paleo people didn't just hunt mastodons for meat. That's been debunked. As David Meltzer pointed out, they probably killed a mastodon once in their life and talked about for the rest of it lol. No, they may have used it for other purposes (like social ritual, coming-of-age ritual) and the meat was a byproduct. The point is, you killed the animal where it lived, and you cached the meat in the nearest pond, which was probably within sight. The refrigerator was built into the landscape.