The sides of the mountain at its base vary in thickness from thirty to a hundred yards.
It therefore follows that this excavation practically occupies the whole of that part of Back Cup island which appears above water.

As to the length of the submarine tunnel by which communication is obtained with the outside, and through which the tug passed, I estimate that it is fifty yards in length.
The size of the cavern can be judged from these approximate figures.
But vast as it is, I remember that there are caverns of larger dimensions both in the old and new worlds.

For instance in Carniole, Northumberland, Derbyshire, Piedmont, the Balearics, Hungary and California are larger grottoes than Back Cup, and those at Han-sur-Lesse in Belgium, and the Mammoth Caves in Kentucky, are also more extensive.

The latter contain no fewer than two hundred and twenty-six domes, seven rivers, eight cataracts, thirty two wells of unknown depth, and an immense lake which extends over six or seven leagues, the limit of which has never been reached by explorers.
I know these Kentucky grottoes, having visited them, as many thousands of tourists have done.

The principal one will serve as a comparison to Back Cup.