On the Canada side of Niagara Falls you can quite literally walk behind them, after an admission fee, of course. A manmade tunnel was blasted behind the falls and has 2 observation areas. Observation is used loosely here; all you’re going to be seeing is rushing water. Visitors are barricaded from the ends of the tunnels.
It’s also not much of a journey; an elevator brings you down to the tunnel, and the tunnel is not all that long. It is cool to be standing directly behind the falls, though. All in all, interesting experience, but something that is likely not a repeat.