I saw three mermaids doing loops and swimming to the sandy bottom. I wasn't sure if I had drowned, or was suffering from narcosis, or if I had perhaps swallowed the worm from a mezcal bottle by mistake. I just had to keep breathing and following Paulo's fins, our guide, because I'd never been scuba diving before and was getting seriously freaked out just for a minute. This was some dive for a first one though, and I really was thrown in at the deep end amongst the tree roots and overhanging rocks of Cenote Manati. It wasn't until I looked on-line later that I finally found the source of my "hallucinations" - large, colourful plastic mermaid fins that fit over both feet of young female "people". What a disappointment!

It is essential to get into the water to see this subterranean fairyland. The Yucatán peninsula is an ancient dead coral reef made of limestone, with jungle now having taken over on top. Acid from the soil has created over 6000 sink holes (cenotes) and an extensive maze of underground rivers, decorated with stalactites and stalagmites created during the last ice age. And although they weren't real mermaids (probably), there are lots of weird creatures down there in the depths that have evolved independently. It's very unique.

As a qualified diver, Helen was able to do more, whilst I was left behind with just a snorkel, mask and fins. I was happy with this arrangement most of the time, particularly when we arrived at Dos Ojos cenote in the early morning, and I was able to have the crystal blue waters all to myself, watching the divers' torches as they followed ropes through the caverns below.

We also went snorkelling in a lagoon filled with a variety of large, vividly coloured fish, and in the sea where we found several huge turtles and a stingray; but I admit to being very envious of the divers who could get closer to the reef for much clearer views of the turtles.

We have spent so much time in the water over the last few days that I wouldn't be surprised if we had grown fins of our own anyway. With a dip in the Atlantic Ocean, this marks the end of our trip across Mexico from the Pacific. The water was a great escape from the heat above, and we expect quite a shock when we shortly arrive back in England for the winter.