so @darlingofdots's awesome Temeraire!universe historian post mentioned the wreck of the HMS Allegiance and I have been thinking about where it is literally all day.

Not just where as in "somewhere in the South Pacific" (because duh) but also, specifically, how deep, and therefore how the wreck would be studied.

Because a lot of archaeologically significant shipwrecks are pretty shallow, since they're the wrecks we can dive to, either on normal air scuba tanks or mixed gas. The Uluburun shipwreck off Turkey, for example, sits between 44 and 61 meters deep, which is right on the edge for air diving. The archaeologists could only be at the bottom for 20 ish minutes at a time, two times per day, with careful decompression timing as they went up to avoid the bends and not-insignificant amounts of nitrogen narcosis at the bottom. Mixed gas goes deeper, 100 meters or so for some of the more available ones. (there's a Phoenician shipwreck off the coast of Malta that's about 110 meters deep, and was excavated by technical divers) Beyond that it's just commercial divers laying oil pipelines with the super $$$ gas at depths of up to 500 meters or so. Anything deeper than that is the domain of submarines and robots.

and really, all of that ^ paragraph is just tangential set dressing that I added because I like shipwreck archaeology, because knowing the Allegiance went down in the middle of the South Pacific meant it was always going to a be a submarines-and-robots wreck. The middle of the Pacific Ocean is uh. deep. but I wanted to find out exactly how deep.

so the map from Crucible of Gold puts the sinking at a little under 50°S and a little over 121°W, which the NOAA bathymetric data viewer says is just about 3000 meters deep

Since that's an extremely boring screenshot, here's the CoG map overlayed on a bathymetric map:

It's actually on a bit of a ridge there! which is why it's at 3000 meters and not deeper.

We do find and investigate wrecks at that depth and deeper these days. The Titanic is at 3800 meters, and it has been investigated extensively (though we also have a recent pretty major news story about why thats still difficult and uh, very dangerous) The USS Samuel B Roberts was found at 6895 meters, and perhaps most relevantly, the search for Malaysa airlines flight MH370 turned up two 19th century shipwrecks at 3500+ meters deep, over 2000 kilometers off the coast of Australia.

One of those wrecks was a wooden ship from either the 1870s or 1880s, and though, being wood, it was pretty badly decayed, its cargo (coal) and metal features (anchor and water tanks) were still extant. On the Allegiance, that would also include her guns and her metal keel (which would probably be the identifying feature TBH, the keel marking her as definitely a dragon transport)

That wreck is probably the best parallel to the Allegiance in other ways, being a wooden sailing ship with a wreck not only very deep but also very remote. It also probably went down due to an explosion, just like the Allegiance. They were common on coal-carrying vessels, and the sonar images showed the cargo was scattered across the seafloor like something catastrophic happened.

The Allegiance would be more remote than its real-world parallel, but anyone looking for it would be hunting for it specifically and would be armed with probably a decent idea of where she was when she went down, seeing as there were survivors who would have been very keen to remember where they were so they could know how close they were to land. Plus, much like the Titanic (though not to the same extent) there'd probably be funding to investigate the Allegiance once found, as she had a part to play in major political turning points on at least three continents. People tend to be interested enough to throw money at that sort of thing.

So, there you have it. It would take a pretty serious effort to find her, though not an impossible one, and once found she'd be investigated by shipwreck robots, which would bring back pictures and samples of her metal remains, with organic matter being mostly absent by the time she was found.