The hues of the Great Blue Hole are striking in their contrast. The sinkhole itself is so deep – 125 metres deep, to be exact ­– that the almost-perfect circle is a dark navy, while the shallow waters of the surrounding reef are a sparkling turquoise. About 70 kilometres from the mainland, the chasm is popular with scuba divers thank to the nurse, Caribbean reef and blacktip sharks that glide through its depths.