a, S. Petronilla on the Via
Ardeatina, S. Valentine on the Via Flaminia, S. Hermes on the Via
Salaria, S. Agnes on the Via Nomentana, S. Lorenzo on the Via
Tiburtina, and fifty other historical structures, owe their existence
to the humble grave which no human hand was allowed to transfer to a
more suitable and healthy place.

When these graves were not very deep, the floor of the basilica was
almost level with the ground, as in the case of S. Peter's, S. Paul's,
and S. Valentine's; in other cases it was sunk so deep in the heart of
the hill that only the roof and the upper tier of windows were seen
above the ground, as in the basilicas of S. Lorenzo, S. Petronilla,
etc. There are two or three basilicas built, or rather excavated,
entirely under ground. The best specimen is that of S. Hermes on the
old Via Salaria.

It soon became evident that edifices sunk in such awkward places could
hardly answer their purpose, on account of dampness and the want of
air and light. Several steps were taken to remedy the evil. Large
portions of the hills were cut away so as to make the edifice free on
one or two sides at least, and outlets for rain or spring water
provided. We have a description of the system of drainage of S.
Peter's, written by its originator, Pope Damasus, in a poem the
original of which, discovered by Pope Paul V., in 1607, is preserved
in the Grotte Vaticane:--

"The hill was abundant in springs; and the water found its way to the
very graves of the saints. Pope Damasus determined to check the evil.
He caused a large portion of the Vatican Hill to be cut away; and by
excavating channels and boring _cuniculi_ he drained the springs so as
to make the basilica dry and also to provide it with a steady fountain
of excellent water."[68]

The Acqua Damasiana is still in use, and has the honor of supplying
the apartments of the Pope. Its feeding-springs are located at S.
Antonino, twelve hundred yards west of S. Peter's. The aqueduct of
Damasus, restored in 1649 by Innocent X., is neatly built in the old
Roman style; the channel is four feet nine inches high, three feet
three inches wide, and runs through the clay of the hill at a depth of
ninety-eight feet. The principal fountain, in the Cortile di S.
Damaso, was designed by Algardi in 1649.

Apparently the works accomplished for the same purpose at S. Lorenzo
fuori le Mura, by Pope Pelagius II. (579-590), were no less important.
They are described in another poem