Extreme Diving By Guillaume Néry

Dean's Blue Hole is roughly circular at the surface, with a diameter ranging from 25 to 35 metres (82–115 ft). After descending 20 metres (66 ft), the hole widens considerably into a cavern with a diameter of 100 metres (330 ft). Sinkholes are created in a similar fashion, although their entrance is generally above sea level. One notable sinkhole is Zacatón in Mexico -- even deeper than Dean's Blue hole -- with a vertical drop of 335 metres (1,099 ft).