Back in my college days my Latin Am history professor noted that the entire Spanish colonial archive was still stored in the original building in Seville, and that it was a matter of concern (that was in the 1970s). You’re right, I believe, a new storage facility was built for conservation of them, but the pile is enormous, so it will be a long time before much is available.
They have begun the work, but can yu imagine 80 million pages of ancient documents that will have to be scanned and cataloged.
And of course, everything is in Spanish.
But they have some remarkable stuff like documents with the signatures of Cortes, De Soto, and Cervantes asking for a job.