The Reichenbach Falls was a waterfall located near the Swiss town of Meiringen. The falls were most famous for the fictional encounter between Sherlock Holmes and James Moriarty, during which the two men struggled and fell over the falls. At first both men were presumed to be dead, but in fact Sherlock Holmes survived.
Arthur Conan Doyle had originally intended for this to be the final Holmes story so that he could move on to other things, but he eventually gave in to fan pressure and began writing more Holmes stories. He was able to explain away his supposed death fairly easily, since in the story, no one had actually witnessed what had happened.