Mexico is one of the most biodiverse countries on Earth. Even though it covers only 1 per cent of the land area of our planet, it is home to over 200,000 different species – 10 per cent of Earth’s bank of life.
Surface water is often scarce in the rainforests of the Yucatan peninsula, Mexico. Yet this region is one of the most biodiverse on Earth.
Cenotes (a type of sinkhole) mark the edge of a massive crater, formed 65 million years ago when an asteroid, measuring some 10 km in diameter, smashed into Earth.
The ocellated turkey (Meleagris ocellata) resides primarily in the rainforests of Mexico’s Yucatan peninsula. Only the male – shown here – has such striking plumage.
The Mexican beaded lizard (Heloderma horridum) is just one of the 707 species of reptile known to exist in Mexico.
The cenotes of the Yucatan peninsula contain remarkably clear water, which has been filtered through the porous limestone above over many thousands of years.
We began filming in the tropical rainforests of the Yucatan peninsula, where accessible water resources can be unexpectedly scarce. Large areas of the Yucatan are devoid of rivers and streams because the bedrock, composed mainly of limestone, is porous. There is a large subterranean source of fresh water, however, contained in a complex, stratified aquifer. Fortunately for the occupants of the peninsula, this underground water source is easily accessible through a series of sinkholes known as cenotes. The cenotes lead into vast networks of subterranean caverns dissolved out of the limestone over many thousands of years and flooded by the clean waters of the aquifer. The Mayans built their civilisation around cenotes, many of which lie in a strange, semicircular arc centred on a small village called Chicxulub. They mark out the edge of a giant crater, formed 65 million years ago when an asteroid 10 km in diameter smashed into Earth. Known as the Cretaceous-Paleogene extinction event (or the K-T extinction), this impact is the most widely accepted theory for the cause of the mass extinction of the dinosaurs.
The Mayans built their civilisation around cenotes, many of which lie in a strange, semicircular arc centred on a small village called Chicxulub.
The water in the cenotes is exceptionally clear because it is filtered slowly through the porous rocks of the Yucatan before emerging after thousands of years to flood this subterranean world. Diving into the clear darkness of these underground wells is a unique experience and a welcome respite from the heat and insects of the forest. As you journey deeper into the cave systems, the sunlight fades to darkness but an abundance of life can still be found. This is typical of what we find in even the most extreme conditions on the planet. Remove light, heat, soil, plants, insects, and even oxygen, and life still thrives. But one ingredient is, as far as we know, absolutely essential for life to exist.
Cenotes contain an abundance of life, and taking a dive into their crystal-clear, yet dark, world is a unique experience.
Water arguably exhibits the most complex behaviour of any known substance. This may come as a shock, because the ubiquitous familiarity of its chemical signature – H2O – is the stuff of the most basic of classroom chemistry lessons. Yet this familiarity hides a deep complexity that we are only now beginning to understand. The complexity doesn’t lie in the structure of water molecules themselves of course: each molecule is made of three atoms – two hydrogen atoms and one oxygen atom. From chemistry lessons gone by, you might recall that the two hydrogen atoms are covalently bonded to a single atom of oxygen. Oxygen has eight electrons around its nucleus, six of which are in the outer shell; these are known as ‘valence electrons’. Four of these are paired together, leaving two lone electrons that would dearly like to pair up with electrons from other atoms.1 Each hydrogen atom has a single electron, which it readily shares with the electron-hungry oxygen, and the result is a molecule of water.
However, this simple tetrahedral arrangement of a central oxygen atom surrounded by two pairs of electrons and two hydrogen atoms is deceptive, because the structure allows for tremendously complex behaviour when water molecules come together in large numbers. And, as we shall see, this unique behaviour may well make water a prerequisite for the existence of life, not only on Earth, but anywhere in the Universe. Perhaps unsurprisingly, given its dominance in our lives, scientists have been attempting to unlock its secrets for over three hundred years.
1 For those who don’t like such anthropomorphic language, it is energetically favourable for electrons with opposite spins to pair up in the available energy levels around a nucleus, and there are four available upper energy levels around the oxygen nucleus for the six electrons to occupy.
MOLECULAR GEOMETRY: Tetrahedral electron pair geometry
We often take water for granted, yet it is a remarkably complex substance, and without it there would be no life, not only on Earth, but anywhere in the Universe.
THE HISTORY OF THE EXPLORATION OF WATER
In the eighteenth century, Europe was full of inquisitive men attempting to unlock the secrets of the natural world, and Henry Cavendish was certainly one of the most eccentric. It is said that he was unable to bring himself to interact with women outside of his family at all, communicating with his female servants by written notes and sneaking around his own house using a specially constructed staircase so as to avoid his housekeeper. His isolation was so extreme that he often kept his experimental findings secret, not publishing or sharing his research with anyone. Such was the extent of this secretiveness that it was only many years after his death that the true breadth of his discoveries became apparent.
Cavendish was a follower of phlogiston theory – a widely held belief that had its roots in alchemy. The theory suggested the existence of an element thought to be contained within all combustible material, called ‘phlogiston’. By the middle of the eighteenth century the theory had been widely discredited, yet Cavendish continued to see worth in it, and attempted to incorporate it into many of his observations. To modern ears, this makes his terminology sound rather eccentric, but his contribution to our understanding of the natural world was extraordinary, not least in his early work on the chemical properties of water.
In a series of experiments, Cavendish produced and isolated a gas by reacting hydrochloric acid with metals such as zinc, iron and tin. In doing so, he became the first person to identify hydrogen in the laboratory. He referred to this new gas as ‘flammable air’ in his poetically named paper ‘Factitious Airs’, published in 1766. Cavendish went on to show that hydrogen reacted with another gas, which he termed ‘dephlogisticated air’, to produce water. This gas was oxygen. His experiments with flammable air eventually