Here's what Google said...
The Dead Sea exists due to a unique combination of tectonic activity, forming a deep, landlocked depression in the Jordan Rift Valley, and a hot, arid climate. Around 2 million years ago, the area became isolated from the Mediterranean Sea, and high evaporation rates caused the ancient lake to shrink, concentrating minerals and salts to extreme levels.
Key Reasons for Its Existence:
Tectonic Setting: Located on the Dead Sea Transform (a fault line between the African and Arabian plates), the basin is a "sinking" landscape that creates one of the lowest points on Earth, preventing water from escaping.
Lack of Outlet: As a landlocked, terminal lake, water enters (primarily from the Jordan River) but cannot leave except through evaporation.
Extreme Evaporation: The intense desert sun evaporates water constantly, leaving behind minerals and salts, making it roughly 10 times saltier than the ocean.
Salt Source: The water dissolves salt from surrounding sedimentary layers and ancient salt deposits (such as the Sedom Lagoon deposits), accumulating over thousands of years.
The lake is technically a terminal lake, and its intense salinity prevents marine life, hence the name "Dead Sea".
ps ... #12 is tough