It was nine o'clock in the morning in Reston, Virginia. Rick Tanenbaum, head of the US Geological Survey, was at his desk checking his email from the overnight hours. He wasn't a believer in Blackberries, and most of it was useless stuff anyway that people just copied him on to make themselves feel important. The sun streamed in through the louvers in the blind. So far, it was just like any other morning.

He was dabbing at his mouth with a paper napkin after finishing his morning bagel, when the phone rang. Shell Oil. Only the highest ranking people in Shell had his private number, so this was a matter of some significance.

"Morning Rick" said the voice on the other end. Pete Prince.

"I think there's something you should know" he continued, before Rick could reply. "There's a giant sinkhole right near Chicago". He froze.

This could be bad news. Very bad news. America's third largest city could be sitting on a hollow in the earth. What if Lake Michigan leaked into it? The consequences could be unimaginable. "How do you know?" he asked slowly. Pete explained the story of the accidental discovery of the gas gusher, and the efforts that were underway to plug it.

"How do you know how big it is?" Rick asked.
"It's got to be tens of cubic miles at least", Pete replied, " just given the pressure. There's no telling how big this thing is".
"We're on it" said Rick, and put the phone down.

Hurried calls were made. Men and machines were scrambled. The USGS hangar at Reston airport became a hive of activity as geological sounding equipment was loaded onto their jet, an aging GulfStream. Explosives were needed too, but there was no time to secure the FAA permits to carry them. What they didn't know wouldn't hurt them.

Within an hour, they were airborne, bound for Gary, Indiana, the closest airport to the site. It was also less busy than Chicago's Midway airport. They set out from there in rented SUVs to the remote site. They parked about a mile from the site, and some hastily-rented drilling equipment was already waiting for them.

A hole was bored 30 feet into the rock, and five sticks of dynamite inserted into it. The sounding equipment was set up a half mile away. "Fire in the hole" came the announcement over the megaphone, and instantly a plume of dust and rock shot up into the air. Two seconds later came the sound report, a sharp crack, and the shockwaves were already traveling through the earth's crust at several miles per second. Almost instantly the screen of the sounder lit up with an image of what was below them.

"Oh my God" came the awed responses. The sound waves had reflected off the wall of the cavern, revealing something vast, spreading out beneath them in all directions, its limits unclear. It was over a mile down at their location, and the top sloped gently downwards the farther you got from the construction site.
"Holy f***ing cow. There's no telling how big this thing is. We're gonna have to go further away."

They moved five miles to the south, and repeated the experiment. The cavern showed up clearly again, this time four and a half miles down. Its outer dimensions were still unfathomable.

Once again they moved south another five miles, and took more measurements. They had to use a bigger explosive charge now, to send shockwaves 15 miles down. The image appeared clearly still, this time showing the cavern wall sloping down steeply, almost to vertical.
"There's no telling how deep this thing is" said Rick.

More measurements were taken from points radiating out from the shaft site, and it was determined that the top of the cavern was shaped like the top of a bottle. The unfortunate construction worker just happened to have broken into it at the point closest to the surface. The questions were just beginning. Just how vast was this thing, and what secrets did it contain?