The four-island circuit from Koh Lanta traces a 60-kilometer arc through the Mu Ko Lanta National Park, a marine preserve gazetted in 1990 to protect 134 square kilometers of coral reef, mangrove estuary, and karst formation along Thailand's Andaman coast. Each year between November and April, when the southwest monsoon retreats and the sea flattens to glass, speedboats and traditional longtail vessels ferry visitors from Saladan Pier to a quartet of islands—Koh Mook, Koh Ngai, Koh Chuek, and Koh Kradan—that form the accessible edge of the Trang archipelago. The route is anchored by the Emerald Cave on Koh Mook, a collapsed limestone chamber accessible only by swimming 80 meters through a lightless tidal passage that opens onto a hidden lagoon ringed by vertical cliffs and rainforest canopy. Koh Mook's western shore harbors Charlie Beach, a crescent of powdered coral sand fronted by shallow reef flats where blacktip reef sharks patrol at high tide. Koh Ngai, five kilometers northeast, rises as a single forested ridge whose eastern flank drops into a fringing reef system mapped by marine biologists in 2003 as one of the Andaman's densest concentrations of hard coral—Acropora tables and brain corals that host parrotfish, angelfish, and seasonal manta rays. Koh Chuek, the smallest of the four, is little more than a sandbar and casuarina grove, but its surrounding shallows hold visibility exceeding 20 meters during peak season, making it a preferred snorkeling site for spotting sea turtles and reef octopuses. Koh Kradan, the southernmost stop, stretches two kilometers of uninterrupted white sand along its eastern coast, backed by cashew plantations and a marine research station operated by Kasetsart University. Speedboat operators departed from Saladan Pier beginning in the early 2000s, when Koh Lanta transitioned from backpacker outpost to established resort island. The four-island itinerary became standardized around 2010, when the Tourism Authority of Thailand designated the Emerald Cave as a signature Andaman attraction and park authorities capped daily visitor numbers at 300 to prevent erosion inside the limestone chamber. Longtail boats, the wooden vessels that predate speedboat tours by decades, continue to run the same route at half the speed, departing at dawn and returning near dusk. Both vessel types observe the 200-baht national park conservation fee, collected at Koh Mook and enforced by rangers stationed at the Emerald Cave entrance. The circuit operates year-round, though June through October brings two-meter swells and reduced visibility that shutter most tours until the monsoon cycle completes.