Charco Verde — the "green pool" — is a small, weirdly emerald lagoon on the western shore of Ometepe between Moyogalpa and Altagracia. It sits in an old volcanic crater half-flooded by the lake, with mangroves and forest around it; the green colour comes from algae and the angled afternoon light against the dark bottom.
A small private nature reserve protects the lagoon and the surrounding forest, with low-key trails through monkey habitat, a butterfly garden and bird-watching towers. The beach next to the lagoon is one of the better swimming spots on the lake — black sand, calm shallow water and a view across to the Concepción volcano. Admission is a few córdobas; an enjoyable half-day stop on a circuit of the island.