Short answers, pulled from the story.
The Black Sea transformed from a freshwater lake into a saltwater sea around 5600 BC following a catastrophic breach of Mediterranean waters through the Bosporus Strait. This rapid rise in sea level occurred within decades rather than centuries according to some researchers.
Over 90% of the deep Black Sea volume consists of anoxic water that does not mix with oxygenated surface layers. A halocline forms below the surface creating a permanent barrier that prevents vertical mixing and isolates the deep waters from atmospheric oxygen.
Mediterranean monk seals were historically abundant but became extinct from the Black Sea basin by 1997 with the last known sightings occurring near Snake Island and Doğankent in the 1950s and post-1990 respectively.
After 1479 when the Republic of Genoa lost control of Crimea the Black Sea became effectively an Ottoman Navy lake where only Venetian rivals like Ragusa could sail merchant vessels.
The flagship of the Black Sea Fleet Russian cruiser Moskva was sunk by Ukrainian missiles on the 14th of April 2022 in the western Black Sea.
Turkey found approximately 30 billion cubic meters of natural gas in the Sakarya Gas Field during August 2020 representing the largest discovery ever made in the Black Sea.