Spain worked silver at Los Almagres somewhere in the San Saba hills in the 1750s, guarded from the presidio on the river, then abandoned the country. Jim Bowie went hunting the sealed workings in 1831 and fought a famous all-day battle with the Comanche rather than quit the search, five years before he died at the Alamo. Texans have hunted it ever since; it is the oldest and biggest lost-mine story in the state.
San Saba River hill country near Menard (TX)
See it on the map