The "Devil's Pool" (or "Devil's Swimming Pool") is a natural rock pool at the very top of Victoria Falls in Zambia where (at certain times of the year) adventurous swimmers can splash around in relative safety just a few feet from where the waters of the Zambezi River cascade over the waterfall and plunge more than 100 meters into the gorge below. The terrifying aspect of such an adventure is heightened for first-time visitors because the pool does not appear to have any sort of barrier (natural or otherwise) to protect swimmers who jump into it from being swept over the edge of the falls.

Here's a video: