Title: Western Asia
Content:  The region is surrounded by eight major seas; the Aegean Sea, the Black Sea, the Caspian Sea, the Persian Gulf, the Arabian Sea, the Gulf of Aden, the Red Sea, and the Mediterranean Sea. To the northwest and north, the region is delimited from Europe by the Turkish Straits and drainage divide of the Greater Caucasus, to the southwest, it is delimited from Africa by the Isthmus of Suez, while to the northeast and east, the region adjoins Central Asia and South Asia. The region is located east of Southern Europe and south of Eastern Europe. The Dasht-e Kavir and Dasht-e Lut deserts in eastern Iran naturally delimit the region from Balochistan and South Asia.
Title: Black Sea Biogeographic Region
Content:  The Black Sea Region is a coastal strip of land 20 - 60 km wide that runs along the coasts of Romania, Bulgaria, and a broader coastal strip in northern Turkey and Georgia. The coastline has rocky bays and sea cliffs, but is dominated by long stretches of low sand dunes and beaches sloping into the Black Sea.
Title: Regions of Europe
Content:  Portugal, Spain, France, Monaco, Italy, Slovenia, San Marino, Croatia, Bosnia and Herzegovina, Montenegro, Albania, Greece, Turkey, Cyprus, Malta and the British territory of Gibraltar ; Adriatic region: Italy, Slovenia, Croatia, Bosnia and Herzegovina, Montenegro, Albania Pannonian countries ; The Panonnian nations are: ; Austria, Croatia, Hungary, Romania, Serbia, Slovakia, Slovenia, and Ukraine Black Sea region ; The Black Sea nations (although some sections lie within Asia) are: ; Abkhazia (de facto state), Bulgaria, Georgia, Romania, Russia, Turkey, and Ukraine Caspian Sea region ; The world's largest lake which forms a section of the Asian-European border has five countries occupying its shore. Iran and Turkmenistan lie entirely within Asia while the following countries are transcontinental and have sovereignty over the Caspian Sea's European sector: ; Azerbaijan, Kazakhstan, and Russia                
Title: Caspian Sea
Content:   The Caspian Sea is the largest inland body of water in the world and accounts for 40 to 44% of the total lacustrine waters of the world. The coastlines of the Caspian are shared by Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan. The Caspian is divided into three distinct physical regions: the Northern, Middle, and Southern Caspian. The Northern–Middle boundary is the Mangyshlak Threshold, which runs through Chechen Island and Cape Tiub-Karagan. The Middle–Southern boundary is the Apsheron Threshold, a sill of tectonic origin between the Eurasian continent and an oceanic remnant, that runs through Zhiloi Island and Cape Kuuli. The Garabogazköl Bay 
Title: Caspian Sea
Content:  1.2% (12 g/l), about a third that of average seawater. It is bounded by Kazakhstan from mid-north to mid-east, Russia from mid-north to mid-west, Azerbaijan to the southwest, Iran to the south and adjacent corners, and Turkmenistan along southern parts of its eastern coast. The sea stretches nearly 1200 km from north to south, with an average width of 320 km. Its gross coverage is 386,400 km2 and the surface is about 27 m below sea level. Its main freshwater inflow, Europe's longest river, the Volga, enters at the shallow north end. Two deep basins form its central and southern zones. 
Title: Pontic–Caspian steppe
Content:  The Pontic–Caspian steppe (sometimes called the "Caspian steppe" or "Pontic steppe") is the steppeland stretching from the northern shores of the Black Sea (the Pontus Euxinus of antiquity) to the northern area around the Caspian Sea. It extends from Dobruja in the northeastern corner of Bulgaria and southeastern Romania, through Moldova and southern and eastern Ukraine, across the Russian Northern Caucasus, the Southern and lower Volga regions to western Kazakhstan, adjacent to the Kazakh steppe to the east, both forming part of the larger Eurasian Steppe. It forms a part of the Palearctic realm and of the temperate grasslands, savannas, and shrublands biome. The area corresponds to 
Title: Black Sea Region
Content:  The Black Sea Region (Karadeniz Bölgesi) is a geographical region of Turkey. It is bordered by the Marmara Region to the west, the Central Anatolia Region to the south, the Eastern Anatolia Region to the southeast, the Republic of Georgia to the northeast, and the Black Sea to the north.
Title: Caspian Sea
Content:  often changed course to empty into the Caspian through a now-desiccated riverbed called the Uzboy River, as did the Syr Darya farther north. The Caspian has several small islands; they are primarily located in the north and have a collective land area of roughly 2000 km2. Adjacent to the North Caspian is the Caspian Depression, a low-lying region 27 m below sea level. The Central Asian steppes stretch across the northeast coast, while the Caucasus mountains hug the western shore. The biomes to both the north and east are characterized by cold, continental deserts. Conversely, the climate to the southwest and south 
Title: Caspian Sea
Content: Hyrcania, ancient state in the north of Iran ; Sari, Mazandaran Province of Iran ; Anzali, Gilan Province of Iran ; Astara, Gilan Province of Iran ; Astarabad, Golestan Province of Iran ; Tamisheh, Golestan Province of Iran ; Atil, Khazaria ; Khazaran ; Baku, Azerbaijan ; Derbent, Dagestan, Russia ; Xacitarxan, modern-day Astrakhan 
Title: Black Sea
Content:   The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia; east of the Balkan Peninsula (Southeast Europe), south of the East European Plain in Eastern Europe, west of the Caucasus, and north of Anatolia in Western Asia. It is supplied by major rivers, principally the Danube, Dnieper, and Don. The watersheds of many countries drain into the sea beyond the six that share its coast. The Black Sea covers 436400 km2 (not including the Sea of Azov), has a maximum depth of 2212 m, and a volume of 547000 km3. Most of its coasts ascend rapidly. These 

Question: the region that stretches between the black and caspian seas?
Answer: The Caucasus Mountains.
Caucasus Mountains.