Most are relics of a bygone era, portals to a distant past.
But today, below the Valley's byways and boulevards, lies a modern tunnel system with a colorful secret: an eye popping venue of kaleidoscopic colors uniting to form a vast underground art gallery.
The murals vary in size and shape.
Some of them stretch for hundreds of yards.
And while many are marked with the typical graffiti you might expect to find on any surface street, most are the work of talented artists.
Tucked away in a variety of shadow-filled nooks and crannies, the murals first came to light on YouTube.
A group of Valley video artists, who prefer not to be identified, have made a hobby of documenting the tunnels. If they know the artists' identities, they're not talking about it. They prefer to marvel at the colors instead.
While ABC15's request to talk with authorities about the safety issues involved was unanswered, it's fair to say that accessing the tunnels is a dicey proposition.
Many of them collect run-off from the city's streets. A sudden downpour could spell disaster as the chambers fill with rushing rain water. Anyone otherwise injured while exploring the tunnels not only endangers themselves but potential rescuers as well.
And then there is the other prospect of meeting someone down there who might have ill intentions. It's a frightening scenario, the perils of which speak for themselves.
Still, such ornate artwork existing in such an unlikely place gives one pause for wonder.
If art is truly in the eye of the beholder, what we saw beneath Phoenix streets is truly something to behold.