We are paddling through a cave, the roof so low that the upper end of our paddles almost scrape it. We paddle past shell encrusted limestone formations hanging down from the roof and as we come around a corner the darkness opens up to a small lagoon. Grey karst cliffs rise sharply from the turquoise waters, up towards the pale blue sky. The cliff tops are dotted with greenery, trees climbing the steep sides, their branches hanging down over the water. We stop paddling and glide slowly out on the lagoon.