Twin Lagoon is a pair of saltwater pools on the western side of Coron Island, separated by a wall of cliffs broken only by a narrow slot. At low tide you can swim through the gap; at higher tides you duck under a small overhang to enter the inner lagoon, which is enclosed by sheer limestone walls hundreds of feet high.
The inner lagoon's water is unusually still and the temperature shifts in patches as warmer fresh water mixes with cool sea water — an effect you can feel sharply when swimming. Bring a snorkel and a dry bag. It's the most magical of the standard Tour A stops, but also the most crowded — go early.