Click here here for a view of the remains of the grotto of Egeria whose elaboration Juvenal laments.

Compare Juvenal's description of the shrine in his day to Ovid's description in Metamorphoses, Book 3.157-162 of the grove sacred to the goddess Diana:

cuius in extremo est antrum nemorale recessu
arte laboratum nulla: simulaverat artem
ingenio natura suo; nam pumice vivo
et levibus tofis nativum duxerat arcum;
fons sonat a dextra tenui perlucidus unda,
margine gramineo patulos incinctus hiatus.

Deep in the woods that fringed the valley's edge, was hollowed in frail sandstone and the soft white pumice of the hills an arch, so true it seemed the art of man; for Nature's touch ingenious had so fairly wrought the stone, making the entrance of a grotto cool. Upon the right a limpid fountain ran, and babbled, as its lucid channel spread into a clear pool edged with tender grass.
--translated by Brookes Moore (at Perseus)