Bonito's most famous sight: a massive limestone cave whose interior harbours a vivid blue underground lake. The colour comes from the depth and mineral content of the water, and in January and February a shaft of morning sunlight reaches the lake's surface at just the right angle, intensifying the blue. Visits are strictly controlled and must be booked through a local agency. Guided groups descend a staircase into the cave; photography is permitted but tripods may not be.