e
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. The roof of the former, like that of the latter, is
supported by pillars of various lengths, which give it the appearance
of a Gothic cathedral, with naves and aisles, though it lacks the
architectural regularity of a religious edifice. The only difference
is that whereas the roof of the Kentucky grotto is over four hundred
feet high, that of Back Cup is not above two hundred and twenty at
that part of it where the round hole through which issue the smoke and
flames is situated.

Another peculiarity, and a very important one, that requires to be
pointed out, is that whereas the majority of the grottoes referred to
are easily accessible, and were therefore bound to be discovered some
time or other, the same remark does not apply to Back Cup. Although it
is marked on the map as an island forming part of the Bermuda group,
how could any one imagine that it is hollow, that its rocky sides
are only the walls of an enormous cavern? In order to make such a
discovery it would be necessary to get inside, and to get inside a
submarine apparatus similar to that of the Count d'Artigas would be
necessary.

In my opinion this strange yachtsman's discovery of the tunnel by
which he has been able to found this disquieting colony of Back Cup
must have been du