Take out a pen and an atlas (Nat Geo, naturally). Draw a line connecting the Philippines, Indonesia, and the Solomon Islands. What do you get? The single richest marine ecosystem on Earth, aka the Coral Triangle. Spanning nearly 2.3 million square miles, it contains 75 percent of all known coral species and more types of fish than there are birds in the Amazon. And yes, like all other reef systems, it too is in danger. Which is where travelers come in. Across the region, eco-conscious lodges like Malaysia's Sipadan-Kapalai Dive Resort serve as important reminders to local governments that their underwater inhabitants are worth more alive than dead. While you won't be solving ocean acidification, your presence—and tourist dollars—can help keep commercial fishing fleets at bay. Sipadan-Kapalai's 51 stilted bungalows sit just off the famed Ligitan Reefs near Sipadan Island, where vibrant coral walls, schools of whitetip reef sharks, and mating mandarin fish draw divers from across the globe (from $650 per person, including meals; sipadan-kapalai.com)).