I Am and I were lucky enough to see the space at the very beginning of the project, as a blank canvas, before the walls were filled with paint and paper. It was the darkest place I'd ever been, at least in New York City, where the street lamps and headlights and the glowing Empire State Building drown out all but a few determined stars.

Pure darkness is the real blank canvas. The white of canvas and paper and gallery walls is the opposite of blank; it's already lit with assumptions. In darkness, you start with nothing.

And so we wanted to work with light, to consider the unique situation of the platform, to construct a work that couldn't exist in the streets or a gallery or a museum because it required the pure darkness, the adventure, the mystery, the discovery. Perhaps most important, the work would require an activator. The spectator is a vital part of the "art" experience, of course, but we wanted something that had to be set in motion. Not just someone to hear the tree falling in the forest, but someone to actually push the tree over.

As we brainstormed, a lot of our thinking focused on the materiality of the station. Perhaps we could install hundreds of LEDs that lit up the station when the trains roared by. Or a platform-wide electroluminescent wire animation activated by a motion detector. But it would have to primarily be an experience, difficult to document, living on through memories and stories.

We flipped through I Am's folder of Eadweard Muybridge animal motion plates and honed in on #374, which depicts two blacksmiths hammering away on an anvil. It was perfect: we'd bring to life the ghosts of the workers of last century right in this unfinished station.

We condensed the series of photographs into six frames, painted each image on a plate of plexiglass, built a box around each frame, and placed a narrow-beam LED light at the back of each box, positioned in such a way that the shadow of each painted frame fell on top of the others' shadows. Then a simple microcontroller, powered by a 9-volt battery, fired the lights off one at a time, re-animating the ever-toiling blacksmiths whenever a switch was flipped.

We showed up with giant plywood boxes, wires hanging out of the back. Workhorse was nonplussed and made us enter first in case we attracted too much attention. Luckily PAC had some collapsible dollies and the boxes were rolled in and up the ladders without too much noise. Wires were connected, and the switch was flipped.

And in the pitch black, deep underground, these ghosts worked silently, unaware of the rest of us. We watched, transported.

And then we flipped the switch and the Shadow Machine was left to stand alone. And who knew how long it would function? Paint and paper fail gracefully, but a swift kick or the pull of a wire could disable the projector forever.

This was the crux of the Underbelly project to us: it was still safer than working in the streets (for the artwork at least), but who knew if it would even have an audience. It was all one big exercise in "if a tree falls in the forest".

Of course, the tree obviously made a sound. You're looking at hundreds of photos that offer plenty of evidence on all aspects of the falling of this tree. But photographs foreground certain traits, and I wonder what is being lost in the retelling of this story.

The most "successful" street artists are those who have a style or a marketable aesthetic that can be sprayed or pasted just about anywhere and look great. They're photographable and rebloggable and reinforce a certain kind of recognizable authorship.

Press about the tunnel focused on the big names that participated, a common journalistic short cut, rather than the success of the works in the space.

Even this book, by its physical properties, foregrounds the paint and the paper. Because how do you effectively communicate a performance or a time-based media sculpture or an interactive audio work on paper?

But the must successful projects in the station could not have worked anywhere else: Posterchild installed original sheet music on a music stand, with two metal cylinders that, when struck, simulated the "ding dong" the NYC subway trains make before the doors close. Two urban explorers contributed an elaborate piece they've asked me not to describe, so at least one undocumented surprise remains for the intrepid visitor.

I Am and I contributed a second project that took into account that every visitor would be carrying a flashlight. When a visitor pointed their flashlight at the piece, it reflected a bird onto the wall. As the viewer walked toward the mirror, the bird raced up the wall until the reflection collided with a red- and orange- and yellow-tinted disco ball and exploded into a fiery light explosion. A flashlight Phoenix.

These are the real ghosts of the Underbelly.