When limestone bedrock collapses, it sinks and forms a pit. Acting as a reservoir, this pit then fills with a mixture of fresh groundwater and saltwater. Eventually, organic matter ends up in the trench, and when it decomposes, it produces a halocline (a cloud of hydrogen sulphide) that separates the freshwater at the top from the saltwater below.
A paradise for the wild scuba divers is this nearly-perfect sphere of deep blue chasm which was formed in the last ice age when the sea level was much lower. As the sea level raised up, naturally the hole developed to be a much deeper abyss.