It goes without saying that this crypt, given its position and structure, belongs to the category of those which are due to the encroachments of the sea. It is at once of Neptunian and Plutonian origin, like the grottoes of Crozon and Morgate in the bay of Douarnenez in France, of Bonifacio on the Corsican coast, Thorgatten in Norway, the height of which is estimated at over three hundred feet, the catavaults of Greece, the grottoes of Gibraltar in Spain, and Tourana in Cochin China, whose carapace indicates that they are all the product of this dual geological labor.
The islet of Back Cup is in great part formed of calcareous rocks, which slope upwards gently from the lagoon towards the sides and are separated from each other by narrow beaches of fine sand. Thick layers of seaweed that h
