Spelunking
Rappel down the side of a limestone cliff, squeeze through damp crevices covered with luminescent glow worms, leap from a subterranean waterfall, and go "black-water rafting" in the underground rapids at Ruakuri Cave, part of the otherworldly Waitomo cave system in New Zealand. The five-hour Black Abyss adventure offered by The Legendary Black Water Rafting Co. is the most challenging of the region's guided tours, with a mix of climbing, rappelling, and cave tubing.
Low-Adrenaline Alternative: For a less-demanding domestic descent, go to Carlsbad Caverns National Park in southern New Mexico. Rather than rivers and waterfalls, the 300 caves here were carved out by limestone-dissolving sulfuric acid, making for a much drier – yet still "deeply" satisfying – experience.
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$59+: Low fares to/from major US cities (one-way) Southwest Airlines
$352+: Discount fares to Asia & Pacific (one-way) CheapOair
$5812+: Around-the-world airfares (round-trip) Air Tahiti Nui
$189/nt+: New Mexico resort stays w/breakfast Perfect Escapes
Australia & New Zealand cruises from $599 Editors' Review
$2145+: Australia & New Zealand 8-nt vacation TripWizards.com