Most of the time when you're geocaching, the cache isn't too terribly hard to find. You may remember in our guide to geocaching, the cache itself was hidden under a grill in a park. That's an easy to find cache. Most are a bit harder. Once in a while, however, you'll find yourself climbing 40ft trees, scaling vertical cliffs (that you probably shouldn't be without safety gear) and crossing raging rivers. At that point, you're into "extreme" geocaching.
Here's one such example of a cache in Denmark where the cache isn't too tough if you can safely navigate the rapid across the river.