The things that are hidden in the Hoover Dam are hidden Ships, Shipwrecks, Train Tracks and Cement Tunnels.
The hidden things are in the water that the hoover dam controls and have been there for years.
Also some people died while building the hoover dam but as far as we know most if not all of those bodies have been recovered.
You can visit and do a self guided tour of some of the hoover dam yourself.
However to get a full tour of the hoover dam you need to pay the fee and have a tour guide take you through the rest of the hoover dam.
When you have a tour guide take you through the hoover dam you will see and discover a lot more of the hoover dam and you'll learn a lot more about the hoover dam as well.
I've visited the hoover dam myself with my kids without a tour guide before and it was still a good experience but later I went back with the kids and paid the fees to get a guided tour of the hoover dam.
The fee to get a complete tour of the hoover dam by a tour guide was well worth it as myself and my kids learned a lot more about the hoover dam and also got to see much more of the hoover dam.
You can drive up to the hoover dam and see some of the hoover dam without paying but it's most certainly worth the fees to get a guided tour of the hoover dam.