There's something irresistible about caves — how they beckon to be explored, reminding us, perhaps, of our prehistoric past. But not all caves are created equal. There are those you stroll through to admire the stalactites — and there are those that require some extra chutzpah to probe. Take cave diving, for instance. Underwater caverns can be breathtaking, but you need some major diving derring-do to take the plunge. Then there's vertical caving and glacier caving, both of which require high-octane rappelling by rope into deep caverns and back up again. Beginning spelunkers, take note, and don't go it alone.