The sea that vanished

The chronoscope

Seventeen million years on one slider

The slider picks a moment. The map shows the extent of the water at that moment over today's borders, rivers and coasts, and Romania stays faintly visible underneath, so you can see what stood above it. The five fixed moments come from two published reconstructions: the Badenian sea of 17–13 million years ago and Lake Pannon of 11 million years ago follow NordNordWest's maps, drawn after the exhibition of the Lower Austrian State Museum; the highest level of the giant lake and the drying that emptied it follow the 2021 reconstruction by Dan Palcu and his colleagues. Between the moments the map dissolves from one into the next, without pretending to know where the shore ran in the years between.

A sea, then a lake

How a sea closes

Some 34 million years ago, at the start of the Oligocene, the mountain chains rising from the collision of Africa with Europe cut the northern part of the old Tethys Ocean off from the rest. The water left north of the Alps, the Dinarides and Anatolia got a name of its own, Paratethys, and a fate of its own: a wide, shallow sea stretching from the Vienna Basin to beyond the Aral Sea, which would open and close several times towards the Mediterranean and the Indian Ocean before it was left, for good, with no way out.

The first moment on the slider catches the sea wide open. Between 17 and 13 million years ago, in the Badenian, the water was as salty as the ocean and connected to the Mediterranean. It covered the Pannonian Plain, Transylvania, Moldavia and the Romanian Plain; of Romania's present territory, only the Carpathians, Dobrogea and part of the north stayed dry. On the 2005 map of the Lower Austrian State Museum, redrawn by NordNordWest, 78.8% of today's country lies under water.

Towards the end of that period, 13.81 million years ago, the link to the ocean tightened so much that the sea began to evaporate faster than it filled. For a few hundred thousand years, until 13.32 million years ago, salt precipitated on the floor of the basins in Transylvania, in front of the Carpathians and in southern Poland, in layers hundreds of metres thick. The Badenian salinity crisis, as geologists call it, is why Romania has Praid, Turda, Ocna Mureș, Sovata, Dej, Slănic, Ocnele Mari and Cacica: the salt in those mines is the water of Paratethys, dried out.

The second moment is the one that made it into the Guinness Book of Records. Around 11.6 million years ago, at the start of the late Miocene, the last straits closed and Paratethys was left a lake. A lake of 2.8 million square kilometres, slightly larger than today's Mediterranean, holding 1.77 million cubic kilometres of water, more than a third of the Mediterranean's volume, according to the calculations of Dan Palcu, of GeoEcoMar and the University of Bucharest, and his colleagues in Russia, the Netherlands and Germany. Its water carried 12–14 grams of salt per litre, about a third of the ocean's and less than today's Black Sea, at 17–18 grams. On their reconstruction of the highest level, 80 metres above the reference level, the lake covers 81.5% of present-day Romania. Eighteen counties lie entirely under water, from Argeș and Prahova to Iași and Constanța. The Carpathians stay above, and on the map so does a piece each of Bihor, Sibiu, Suceava and Alba.

In a lake that large and that lightly salted, life went its own way. Clams and ostracods found nowhere else, seals, dolphins and small whales, raised in that closed water. The smallest baleen whale known, Cetotherium riabinini, lived here: the skeleton found in Ukraine is about three metres long.

Your county

How much of your county was sea floor

The same maps, cut along today's counties. Each bar shows how much of the county's area lies under water at that moment. The figures carry a tolerance of ten to fifteen kilometres at the shore, which is the precision of the published maps and of their fit over today's map, so a county "entirely under water" may keep a corner of hill above the surface, and one at 5% may in fact be dry.

The drying

The lake that emptied by three quarters

A lake with no outlet lives on rain and rivers. When the climate dried, between 11 and 7.5 million years ago, Paratethys went through four crises, one after another. Palcu and his colleagues dated them on the cliff at Cape Panagia, on the Taman Peninsula, where the layers of the lake floor read like the pages of a book, with the magnetic polarity of the Earth as the calendar. The first crisis brought the water down by about 230 metres. The last and worst, named the Great Khersonian Drying, began 8.3 million years ago, worsened between 7.9 and 7.65 million years ago, and lowered the level by more than 250 metres.

At the lowest level, the water gathered into a salt lake in the basin of today's Black Sea, at 28–33 grams of salt per litre according to the model, and into a few freshened lakes at the margins: in the Pannonian Plain, in front of the Carpathians, at the Kuban and the Terek, in the Caspian. The salt stayed in the middle and the fresh water at the edges, exactly as happened to the Aral Sea in recent decades. On the land that appeared overnight, geologically speaking, steppes and forest-steppes spread and joined Central Asia to Europe. Steppe animals moved west through them, and pollen in the European layers shows peak aridity between 8 and 7.5 million years ago, precisely when the lake was smallest.

Then, 7.65 million years ago, the water came back. The Maeotian transgression refilled the basins, but the lake was never whole again. From here on, the history of Paratethys is the history of its breaking into pieces.

What is left

Where the sea still shows

Transylvania was the first to come out. The Vienna and Pannonian basins broke away from the rest of the sea 11.6 million years ago, and the Transylvanian Basin followed about 300,000 years later, 11.3 million years ago, as the Eastern and Southern Carpathians rose and closed it in. The date comes from the section at Oarba de Mureș, in Mureș County, where volcanic ash beneath the transition layers could be dated with argon. That is why the two maps from roughly 11 million years ago contradict each other about Transylvania: the Austrian map shows it already dry, while Palcu's reconstruction keeps it under water at the lake's maximum. Both can be true, a few hundred thousand years apart.

Lake Pannon died slowly, filled by the deltas of the Danube and the rivers of the time, which advanced through it by about 67 kilometres every million years. Its own fauna disappeared around 4 million years ago, when two deltas advancing towards each other met in the south-eastern corner of the plain. The Dacian Basin, the water in front of the Carpathians, held out longest on Romanian territory: in the late Pliocene and early Pleistocene it too filled with sediment, and its rivers began to spill over the sill into the Black Sea. The Black Sea and the Caspian are what is left of Paratethys.

The rest is visible to the naked eye, if you know what to look for. The salt at Praid and Slănic. The concretion-bearing sands in the Buzău hills, where a river once ran towards the lake. The oil and gas of the Romanian Plain and of Transylvania, formed from what lived and died in that water. The Romanian Plain itself, flat as a table, because it is the floor of a basin filled to the brim. And underneath it all, in the layers every water well in the south and east of the country passes through, the shells.

Of today's Romania's 237,432 square kilometres, 193,442 lay, at one point, under the largest lake that has ever existed.Computed on the Palcu et al. 2021 reconstruction of the maximum level, in an equal-area projection; see the method.