I’ve read a lot of comments deriding Robin’s mom and the other archaeolegists for being foolish, throwing the books in the water instead of trying to save their own lives. Ink and paper dissolves in water, so unless someone fishes those books out immediately, all their efforts will create is a papery mess on the bottom of a lake.
However, I believe that their heroic sacrifice did manage to save the books. Robin’s mother didn’t die foolishly.
After they’re done with the bombing, the marines went onto land one last time to make sure there were no survivors. And we know that Aokiji was there during the buster call. He was the only one there with the power to save the books, and as it happened, he had a change of heart during the buster call, saving Robin.
After burning down the island and confirming there were no survivors, the World Government wouldn’t have any reason to send more ships there. So if Kuzan was the last one to leave, or he just returned later, he could have frozen the lake without anyone noticing.
And one of Aokiji’s attacks are called “Time Capsule”. C’mon, that’s gotta mean something. He even used it during the buster call, on Saul.
We know that Aokiji’s ice can last a long time. On Punk Hazard, it managed to stay frozen for two years, even though the other half of the island was on fire. Of course, his devil fruit may not have been as powerful back then, but it wouldn’t have needed to. Just a month or two, and anyone interested in the incident, like, say, the revolutionaries, would have plenty of time to show up and discover the lake. With care, it would be possible to hack up the ice and save the books.
*Theory by Birdy