Dean's Blue Hole is the world's second deepest known salt water blue hole with an entrance below the sea level. It plunges 202 metres (663 ft) in a bay west of Clarence Town on Long Island, Bahamas.

Based on this passage, dean’s blue hole is one of deepest? Let's think fast. Stream of consciousness:
To answer this question, we should know that: Dean's Blue Hole is the world's second deepest known salt water blue hole with an entrance below the sea level. So the final answer is salt water blue hole with an entrance below the sea level.