Blue holes are developed during past ice ages, when sea level was as much as 100–120 metres (330–390 ft) lower than at present. At those period, these formations were subjected to the som erosion caused by rain and chemical weathering typical in all limestone rich terrains. This ended once they were submerged at the end of the ice age.Most blue holes contain both freshwater and saltwater. The halocline is the position in these blue holes exactly where the freshwater meets the saltwater and where a corrosive reaction takes place that eats away at the rock. Over time this can generate side passages, or horizontal "arms", that stretch from the vertical cave. These side passages can be quite long; e.g., more than 600 metres (2,000 ft) in the case of the Sawmill Sink in the Bahamas.
