The "obvious disturbance" concern only makes sense if we take it for granted that a treasure was buried at some incredibly deep depth. It didn't have to be.
Now, let's instead say that I went out in the woods someplace. I pick a spot a good distance from any trails. I'm looking for an area completely loaded with underbrush on rocky, badly uneven ground that's completely unsuitable for cultivation and buiding. I uproot a few shrubs, dig a hole a few feet deep, bury the treasure, and replace the roots. I don't fell any trees, I don't use any rigging equipment, and I certainly don't dig any holes to China. The work is done very quickly and quietly in an area that's of no interest, and no other nearby indications of buried treasure are left. What are the odds of someone going out there to find my treasure? None, unless it's one of my people or a person that one of my people told. And what are their odds of finding it without modern equipment? Remote at best. Even with modern equipment, you'd need a bit of luck and/or some pretty detailed information.
But these guys were smart, and they could certainly do better. So can I. Several moments after coming up with my first plan, I came up with my second one: I would hide my treasure in plain sight, and I'd have a person guarding it. If I have a group of people that I can trust to bury this thing and keep their mouths shut, I have at least one person that I can trust to watch over it for me. (And if I did not, I would do the job myself.) Instead of inscribing rocks and digging immense holes, I'd have a man clear some land and establish a homestead. He'd build a house and a barn, raise animals, grow crops, and do exactly what everyone else in the area was doing at the time. He'd have his family with him; if that was impossible for some reason, he'd explain that he was saving money and getting everything in order before sending for them - again, just like everyone else.
A farmstead like this needs a well, perhaps more than one. That would require a hole. Sometimes you have to dig a few before you get a really good well. My man would dig as many as he had to until he had such a well. And my treasure? It would be at the bottom of one of those unsatisfactory wells, or all of them, or even the good one.
My treasure is now a buried secret with an undercover watchman, and because the local yokels have no reason to suspect anything different or to look for treasure in the first place, that's exactly where it would stay until I was ready to come for it. No stones required engraving, no artifical beaches were constructed, and no 130-foot-deep holes were dug. All that happened was a farmer did what farmers do.
Suppose that something bad befell my employee? Perhaps he got sick and died. In this case, my treasure's still safe, although it's no longer being watched. I'd have another man buy his farm when word got to me. But say that the worst happened, and my man turned on me and decided to recover the treasure for his own use? I'd like to think that I picked a guy that wouldn't do this to me, but temptation always remains an issue. If this was a concern to me (and it should be), I'd get one or two adjacent lots and have my people on those too. They would not be known to my watcher, and they would not know why he was there and what he was doing. They would only know that I wanted them to keep an eye on his land. He might very well abscond with my treasure, but I'd know who took it and possibly where they went. I would not know this if my treasure was buried on an uninhabited (or even sparsely inhabited) island with no one watching it and the equivalent of a giant neon sign telling people to look for treasure.
I'd need a trustworthy man or three to do this for me, but I'd need a legion of trustworthy men to carry out what allegedly happened in most of the legends. And again, if I didn't have a man that trustworthy and it was that important, I'd do it myself.
This would not have to be done in the middle of nowhere. I could carry this out nearly anywhere. There would be fewer people around to see what was happening in remote locations, but I might be happier with lots of people around. A known person digging wells would not raise any suspicions, but unknown people digging up his land certainly would. And again, I have a person or people keeping an eye on things for me. I'm personally much more comfortable when other people are watching my things for me when I'm not around.
These are of course very simple methods for hiding a treasure, but I tend to favor simple answers. Digging a gargantuan hole with flood tunnels and all the other stuff is simple only in theory.
A block left to show where to dig and 
