Sawa-i-Lau is an odd limestone island amid a string of high volcanic islands. The underwater limestone is thought to have formed a few hundred metres below the surface and then uplifted over time. Shafts of daylight enter a great dome-shaped cave – 15m tall above the water surface – where you can swim in a natural pool.
With a guide, a torch and a bit of courage, you can also swim through an underwater passage into an adjoining chamber. The walls have carvings, paintings and inscriptions of unknown meaning.