The cap had been built for over a month now. There were two layers to it, in fact. There was a six foot-thick reinforced concrete cap at the surface, covering the entire mouth the 100 foot-wide shaft. 200 feet below was a second cap, also six feet thick. They had been built with exploration of the cavern in mind, so each had a set of massive doors, not unlike nuclear blast doors, built into it. The dual-cap arrangement, as well as providing more safety, also provided an airlock for entering or leaving the cavern. The temporary cap, plus the remaining rock at the bottom of the shaft on which Charlie's digger had originally sat, were blasted away, to provide a clear path down into nothingness.
The echoes from the falling rocks hitting the bottom of the cavern had provided scientists with their first estimate of the cavern's vast depth. The time it took for the sound to return was measured precisely, but the speed of sound varies with air temperature and pressure and since these were unknown variables towards the bottom, only a rough estimate could be made. It was at least 25 miles deep. This had left scientists around the world dumbfounded. How had it not filled with lava? What could be down there? They were very, very glad of the doors in the caps. Exploration of this inner space would be possible.
A panel was hastily convened. Consisting of over 50 scientists from over ten universities as far apart as Sydney and New Mexico, the USGS, the Department of Energy and even NASA, it was officially entitled Deep Earth Exploration Panel. This had the rather neat acronym of DEEP.
NASA's Jet Propulsion Laboratory immediately set to work building probes that would take scientific measurements inside the cavern. There would be two. A simple dropsonde would parachute right down to the bottom, to measure pressure, temperature, depth, and gas composition. A second sophisticated imaging platform would bounce millimeter-wave radar waves off the cavern walls, to provide a detailed 3D map of the inside of the cavern. It would also have cameras and lights, so that the scientists could see the walls, at least within a mile or two.
At last the day came. All the members of DEEP were huddled around banks of monitors inside the makeshift command center, which consisted of three trailers, generators, satellite links, and many miles of cables. TV news crews and the world's media waited impatiently outside, with three lucky news crews allowed into the operations room. The probes were in the airlock, and the massive doors in the bottom cap were open. They dangled over the void, like tightrope walkers between skyscrapers.
The dropsonde, christened Edna after Charlie's wife, would parachute straight down, theoretically landing within a mile or two of where he lay at the bottom. It also carried a small photo of them on their wedding day, that she wanted to be near him.
Edna was released, and fell into the blackness. Instantly, gas composition readings came up: 80% methane, 19% sulfur dioxide, and 1% trace elements. It fell quickly, and the temperature and pressure readings built up. Within 20 minutes, it reached the bottom, and the readings were astounding: 52,000 millibars, or 52 atmospheres, of pressure, and 600 degress Fahrenheit. The volcanic scientists among the group nodded expectantly. The others exhaled slowly, or let out a quiet whistle.
But, there was better to come. The second probe, named Priscilla after Charlie's daughter, was lowered through the huge trapdoor. A specially-modified crane dangled it on a mile of steel rope. The radars were then turned on. Instantly, a 3D false-color image of the cavern's interior came up.
"Oh my god" whispered one of the scientists, while the others watched in hushed awe. The room was darkened, and a large monitor in the center showed a 3D rendering of the cavern from the probe's point of view, with others showing the entire thing from the outside. Its true, vast dimensions became apparent.
The cavern was shaped like a giant teardrop, being almost was wide at the bottom as it was high. The inside surface was fairly regular in shape, with no huge topographical features to speak of. Instantly, speculation began to flow about how it formed, and why it had not collapsed into itself long ago. More than one person felt queasy at the thought that they were sitting on such a thin crust of rock on top of such a long drop.
Then one sharp-eyed scientist from JPL spotted something. About 10 miles up one wall was an opening, maybe a mile and a half wide. Miles below, at the bottom, was what looked like debris from a rockfall. Instantly, everyone's attention was on the opening. "What is it?" asked one. "How did it get there?" asked another. Then someone else voiced what everyone else was thinking: "What's in there?".
Then, Dan Robbins, the gray-haired head of NASA's JPL, spoke up. With one finger he tipped up the brim of his tan-colored cowboy hat, as he always did when he had something important to say. "Gentlemen, there's only one way to find out what's in there. We're going to have to send humans there.".