Picking through the 'bleached bones' of leviathan

Dorrik Stowe's hunt for Tethys, a lost ocean that once dominated the planet, takes leads him from the Dolomites to the Himalayas

The Tethys Ocean once dominated the Earth. Its vast waters were party to many of the most dramatic episodes in the story of our planet. It played host to a plethora of fascinating organisms, saw mass extinctions and nurtured the ensuing birth of new life forms. It endured its own secret drama of submarine quakes and powerful currents, at one point trembling under evening skies lit by the outpourings of a super-volcano that exploded along its northern margin. For long ages, its quiet central gyres were more desolate and empty than anything seen in today's world. Then, 6 million years ago, it disappeared.

I have been on the trail of the Tethys for two decades, seeking out its changing shoreline and clues as to its demise. Traces are scattered in rocks from the high Himalayas to the Deccan traps of western India, from the Middle East oilfields to the Straits of Gibraltar, from northern Africa to the Mediterranean islands. Though patchy, the evidence can be systematically collated and the ocean scientifically reconstructed. Hints of its existence began to come together with Alfred Wegener's pioneering work on continental drift at the beginning of the 20th century. But much of the effort to locate fragments and reveal the story of its evolution and destruction has happened in the past 40 years. Recreating the Tethys is rather like attempting a global jigsaw puzzle from which many pieces are missing. Some of the pieces, especially those that shine light on its death, I have gathered from my research. Many more rely on the work of other scientists and colleagues, too numerous to mention.

In Greek mythology, Tethys is a sea nymph and is the wife of Oceanus, a mighty river that stretched around the world. Her name captures something of the greatness and elusiveness of this lost ocean. The Tethys was born about 5 million years ago with a gigantic indentation that straddled the equator and cut deep into the eastern side of the supercontinent Pangea from the global ocean Panthalassa. Shortly after it was formed, the world experienced its greatest mass extinction. Up to 95 per cent of species were wiped out, with the death toll heaviest among marine organisms. The Tethys, with its broad shallows and tropical climate, was the perfect place for life to recolonise the devastated planet. Fossil evidence reveals how new life forms flourished in its waters.

New types of microscopic plant life and zooplankton that fed on them took hold, their skeletal remains building up over millions of years to leave the white cliffs and chalk uplands so common today. Diatoms proliferated, building their silica skeletons into beautiful, intricate shapes that led to the formation of sharp black nodules of flint. A deep-water, sponge reef spread across the northern Tethys, stretching from what is now Spain to the Romanian Black Sea coast to which oysters, clams and rudist bivalves attached and into which they burrowed, while free-swimming shellfish, such as magnificent coiled ammonites, explored its flanks. Voracious crabs, lobsters and meat-eating snails prowled the sea floor. Crowning a highly complex food web were advanced fish, such as sharks and marine reptiles.

Crocodiles and turtles appeared for the first time.

In the Dolomites of northern Italy, I have found evidence of perhaps the high water mark of the Tethys's global impact. Early in the Jurassic period the ocean broke westwards, advancing through the former Pangean supercontinent and forming an equatorial seaway that extended from Mexico to the Far East. As the Tethys split the continents asunder, rift valleys opened and sea-floor spreading ensued. The ocean waters flooded into the once desert-baked interior of Pangea. This episode left its mark on the Dolomites, where the deep-marine basins that opened up were quickly filled with a jumble of fallen reef blocks and thick volcanic sands. To the south, in sunflower-clad hills near San Marino, my students have documented further echoes of this past, revealing signs in the rocks of the gradual drowning of the submarine landscape. These have allowed them to recreate a vista of the Tethys similar to the Bahamas.

But the golden age of this tropical ocean was soon interrupted. The end began as opposing continents - Eurasia to the north and Gondwanaland to the south - began inexorably to override the Tethys sea floor. The ocean, though, did not slide quietly into oblivion. As submarine trenches formed along its margins and ocean crust was consumed, so mountains were thrust upwards along the adjacent landmasses. The once peaceful ocean rim was beset by earthquakes and volcanic fury. Through the millennia that followed, these newly formed peaks, from the Atlas and Betic chains to the Himalayas, were savagely eroded, feeding into great rivers that discharged their detritus into the Tethys.

As a specialist in the processes that operate on the deep-sea floor, this period of Tethys's history - when the ocean became racked by its death throes - excites me most. In one of my research programmes, we are examining the sandy fill of deep submarine ravines that formerly scoured the Tethys slopes between 30 million and 40 million years ago, as the big squeeze got under way, but that are now exposed on land along the coast of northern Africa and in Sicily. When, as leader of a research workshop on these processes in the delightful coastal town of Cefalu in Sicily, I stood with some 50 specialists in one gigantic canyon and reflected on how we were dwarfed by the deposit of a single flow.

High in the Himalayan foothills of northern Kashmir and Assam, I have located similar deep-sea Tethyan deposits. On my first visit to Kashmir, I believe I came close to being the centre of a hostage drama. On the day of departure, I was met by two men at the hotel and driven for two hours in a direction away from the airport. After a third man had squeezed in beside me, we stopped and I was escorted into woods near the disputed border with Pakistan. I was then released. I do not know the purpose of this "visit" nor the reason for their change of heart. I was nevertheless captivated by Kashmir's beauty and remoteness. Despite the risks, I am planning a second expedition with Asian colleagues. In some ways it is as untouched as the deep-sea floor, ripe with untold stories.

As slithers of the disappearing Tethys ocean floor became plastered onto the advancing continents, some were thrust to ever more dizzying heights.

Even the 8,500m summit of Mount Makalu, a neighbour of Everest on the Nepal-Tibet border, displays a remarkable section of Tethys crust. More accessible is a fragment that survives on Cyprus. Here the rocks tell a tale of a thriving, colourful ocean as every bit alive as those of today.

They also tell us about the Tethys's catastrophic end.

Mount Olympus, the legendary home of the gods at the heart of Cyprus, is also the core of the Troodos ophiolite, as these pieces of ocean crust are called. It is one of the few places where you can walk along the roadside and view rocks that sequentially once made up the outer crust of the Tethys right down to the deep interior of the Earth's mantle. On other parts of the island I have crossed acres of rugged basalt lava flows that were once ocean floor. Large depressions mark its surface, coloured brown by hot fluids that pumped thousands of tons of dissolved iron, manganese and other metals into the cold waters of the Tethys, the equivalent of mid-ocean ridge black smokers. Such finds are rare and beautiful indeed.

The plight of the Tethys, squeezed by the continents and smothered in sediment, took a dramatic turn for the worst. First India collided with Asia, and Africa pushed north, cutting it off from the global ocean to the east. Then, about 6.5 million years ago, tectonic forces lifted a landmass from southern Spain to North Africa, finally stopping its link to the Atlantic in the west. The once mighty ocean was marooned. The Tethys rapidly began to dry out in the arid climate until all that was left was an irregularly shaped series of basins roughly similar to the present day Mediterranean and Black seas. As the water evaporated, the concentration of sea salts increased until marine life could no longer survive. Mass death followed. There was no let-up in the burning heat of the blistering white basins.

Life as we know it was entirely absent where once there had been profusion and diversity. The remnants of the Tethys evaporated many times before being temporarily refilled from the Atlantic in tremendous cascades across the Strait of Gibraltar. I have walked with my students in the badlands of southeast Spain and across the lower slopes of southern Cyprus, where whole hillsides are formed of gypsum that crystallised as the Tethys evaporated.

We pause for thought before the bleached bones of a dead ocean.

Some 5 million years ago, after the turmoil of successive flooding and drying was over, the Tethys disappeared for ever. In its place the waters of the Mediterranean gained their present nature and azure calm. Barely a trace of the great ocean was left, save in the rocks.

Often, as I take research cruises out into the oceans, I wonder to what extent earth history is repeating itself. On one expedition to the central Indian Ocean, we drilled through nearly 1,000m of sediment that had eroded from the high Himalayas and been spread out across the sea floor nearly 5km below the surface and 5,000km from their source. In a remarkable cycling of Earth's elements, grains of sand and silt that trickled through my fingers had once been deposited on the floor of Tethys Ocean, were buried and turned to rock, before being forced to the top of mountains and returned as dust to the floor of a different ocean.

Dorrik Stow is professor of ocean and earth science at Southampton University and author of Encyclopedia of the Oceans , published by Oxford University Press, £25.00.