I've visited the Nevada Test Site. Our fossil drilling history has given us an unparalleled ability to bore straight holes eight and twelve feet in diameter (standard sizes on the Site) for thousands of feet down. Start anywhere in the country and rill an eight-foot hole down through any sedimentary strata to basement rock, and then keep going for another few thousand feet. drop anything you want in there and allow room for a few hundred feet of sealing concrete before you reach the top of the basement rock layer, and you have a time capsule that will stay there for geologic eons.
We could dispose of our spent nuclear fuel that way, but we wouldn't want to. We would be wasting a large amount of usable fuel.