A very significant fraction will be natural history specimens. A long time ago, I did the London Natural History Museum's MSc course in biodiversity. For one of the modules, in curation, we visited the museum's external storage site, a warehouse complex in southwest London. I barely have the language to describe the scale of the collections there except to say it was on an Indiana Jones 'Raiders of the Lost Ark' level and there were still vast unprocessed archives of material from biological expeditions dating back to the 1800s. There were entire slowly decomposing cetaceans in enormous metal chests, aisles of taxidermy zooming off to infinity, floors and floors of fossils, large to tiny. At the museum's main site the entomology collections alone have over 34 million items.