The question of liability for injury suffered by a cacher in search of treasure falls under the legal area of negligence. Typically, in order for someone to be held negligent, they must have some legal duty owed to others, must have breached that duty, and the breach must have been the proximate cause of the injury suffered by the injured party. Along with that is a principle known as Assumption of the Risk. Jeremy's disclaimer makes it clear that anyone relying upon the information on the site assumes all risks involved in seeking a cache. Jeremy's adequately protected.
As for liability of others, if the cache is located on publicly owned land, the governmental entity owning the land would be protected pretty well by sovereing immunity (you can't sue the government, unless the government agrees to be sued).
For caches located on private land, typically a landowner is liable only for forseeable risks (assuming that the injured party is not a trespasser, in which case the landowner is completely protected, unless there exists some "attractive nuisance" such as a swimming pool which might attract children "of tender years" who don't know any better than to go drown in your pool). Plodding through the woods is commonly recognized as entailing some risk to the explorer, the nature of which risks are more than likely unforeseeable to the landowner.
As for liability to the hider of the cache, unless it was intentionally placed with some idea toward harming the unwary, there would be no liabilty.
That, in a nutshell, is the law in every state. As for other countries, I have no idea, but would assume that, for the most part, it is the same.
BTW, I am an attorney and regularly handle personal injury cases. A cache hunter who falls into an abandoned mine shaft? I wouldn't touch the case.