The Pannonian Sea was a shallow ancient sea, where the Pannonian Basin in Central Europe is now. The Pannonian Sea existed during the Miocene and Pliocene epochs, when a 3–4 km (1.9–2.5 mi) depth of marine sediments were deposited in the Pannonian Basin.
During its first historical phase, the Pannonian Sea had a western connection with the Mediterranean Sea through the territories of the modern Ligurian Sea, Bavaria, and Vienna Basin. Through the Đerdap Strait, the Pannonian Sea was linked to the Paratethys in the Wallachian-Pontic Basin. The Pannonian Sea was also attached to the Aegean Sea through the modern Preševo Valley.
Due to its diverse history the salinity of the sea often shifted. The decrease of salinity resulted in endemic fauna.
The Pannonian Sea existed for about 9 million years. Eventually, the sea lost its connection to the Paratethys and became a lake permanently (Pannonian Lake). Its last remnant, the Slavonian Lake, dried up in the Pleistocene epoch. The remnants of the former islands of the Pannonian Sea are the modern Pannonian island mountains (Mecsek, Papuk, Psunj, Krndija, Dilj, Fruška Gora, and Vršac Mountains).
- Paratethys – A large shallow sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in Central Asia
- Pannonian Basin, also known as Pannonian Plain
- Piemont-Liguria Ocean – A former piece of oceanic crust that is seen as part of the Tethys Ocean
- Iron Gates – A gorge on the river Danube between Serbia and Romania
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