This is a weird but wonderfully beautiful place about 2 hours south of Cancun on the Riviera Maya. The worlds longest underground river comes out around 1k from the Caribbean sea and creates a amazing lagoon/slow moving river. You can walk all the way to the where the river comes from the floor, in deep rain forest and jump on to inflatables and be lazy rivered down to the main, wide estuary area. The first part of this journey is through a thin path cut in to the mangrove which soon opens up to a wider river. All along the way there are jump off points with people hurling themselves off the cliffs in the water (guards are there!). The snorkelling is good, but if you have ever been places like the Great Barrier, or Ningaloo, you might feel that you have seen it before. We did see a massive Barracuda and loads of other fish but sometimes there were big gaps where there were no fish. This is my only criticism. Entry is something like $70US and food and drinks are included in this. Other activities that with additional costs include swimming with, and getting 'pushed' by Dolphins, swimming with Manatees and walking underwater in specially weighted suits. I would definitely recommend for a day out if in the area.