"In all, from the end of World War II to the present, some 700 cavities consisting of tunnels, galleries, caves, secret passageways are known so far. Greek caves, Roman and Bourbon tunnels, catacombs and natural grottos make up a total of a million square meters of underground space. Recent and continuing explorations by a group of enthusiastic experts and devoted cave explorers indicate that much more remains to be discovered. Right beneath our feet there remains, conservatively, another two million square meters of unexplored, undocumented spaces."

Reconstructed Greek burial chamber

I was in but one (!) of 700 such spaces, many as big as churches. You could have a large worship service of devout troglodytes down here—or you could hide for weeks (and longer) as people did to escape the falling bombs in WW II.

Think how it all came about. The Greeks built the initial aqueduct, running water down from sources on the slopes of Vesuvius and Somma to their new city of Neapolis, filling cisterns that supplied water to wells from whence it was hauled up to meet the needs of all those toga-clad surface dwellers sitting around sipping a fine resina and bemoaning the fall of Athens. Then the Romans built their spectacular 70-kilometer conduit to bring water from the Serino river to Naples, Pozzuoli and Baia, where it filled the huge "Piscina Mirabilis" or "Wonderous Pool" to provide water for the Imperial Fleet at Miseno. That served the city through the Middle Ages and into the 1600s when the Spanish expanded it, and that lasted until the 1880s when a series of devastating cholera epidemics led to demolition and reconstruction of large sections of the city, work that included the construction of a modern aqueduct.

Along the way—and this is how those 700 caverns got dug over the centuries—you built your house by first getting the land and digging down into it for building material, the yellowish volcanic rock called "tuff." Imagine quarrying out an upside-down funnel beneath your property. The narrow spout will become the well that supplies water to your future house that is now growing up around that central shaft as you haul more and more material up. Then you angle out the sides of the chamber to form the real funnel and then dig straight down until you need no more rock to build with. That huge space is the cistern. Thus, you have house, well, cistern and, with a bit more digging, you run shafts over to tap into the main aqueduct. River to aqueduct to shaft to cistern to well to you. As simple as that—considering that you used nothing but hand axes, picks, and sweat. (Marxist grammarians will note that "you" in the last few sentences almost certainly means someone else.)

The cisterns, wells and shafts are in addition to (!) the underground spaces unrelated to the need for water, such as historic catacombs and strategic tunnels. (The 18th century Bourbons had such an escape tunnel from the main royal palace downtown to the palace on the Capodimonte hill, just in case a revolution broke out.)

The current work beneath Piazza Cavour is the labor of love of Clemente Esposito, Fulvio Salvi and a band of volunteer urban spelunkers of the organization, Napoli Underground, who have already opened a small museum at the surface with a display of maps, tools, and recovered artifacts. The eventual plan is to recreate (!) beneath the surface examples of what you would find if you could actually descend at one end of the city and stroll underground to the other side, That, of course, was never possible, but today the kilometers of shafts and hundreds of empty cisterns are dangerous, dark and full of debris dumped in during and after WW II.  Thus the volunteers are building Greek burial chambers, Christian catacombs, chambers with Priapic cult symbols (that one is already done, but I blush to show you the photo I took!), as well as 20th-century air-raid shelters, the walls of which are etched with the graffiti of the bored, the patriotic, the frightened.

Speaking of frightened—back to what I was reading, The Time Machine by H.G. Wells:

"…I had to clamber down a shaft of perhaps two hundred yards…I was speedily cramped and fatigued by the descent… One of the bars bent suddenly under my weight, and almost swung me off into the blackness beneath. For a moment I hung by one hand, and after that experience I did not dare to rest again…"