The reservoirs of Carthage were of enormous extent, and some
of these remain to this day and are the wonder and admiration of
travellers. They were subterranean, and were cut from the solid
rock, the stone extracted from them being used for the walls of the
buildings of the city. Pillars were left at intervals to support
the roof, and it was calculated that these underground lakes --
for they were no less -- contained sufficient water to supply the
wants of the great city for at least six months. These vast storing
places for water were an absolute necessity in a climate like
that of Northern Africa, where the rain falls but seldom. Without
them, indeed, Carthage would have been at the mercy of the first
army which laid siege to it.

The greatest pains were devoted to the maintenance of the water
supply. The rainfall from the roofs of the temples and houses was
conducted to the reservoirs, and these stores were never drawn upon
on ordinary occasions, the town being supplied with water brought
by aqueducts from long distances among the hills. Here and
there openings were cut in the rock which formed the roof of the
reservoirs, for the admission of air, and at a few points steps
from the surface led down to the water. Iron gates guarded the
entrance to these.

Nessus and his friends had the evening before unfastened one of
these gates. The lock was old and little used, as the gate was
placed rather to prevent children and others going down to the
water than for any other purpose, and the Arabs had found little
difficulty in picking the rough lock.