'At great cost, Helios has peeled the onion even deeper. We've consolidated an aerial-seismic mosaic of overlapping earth images. We took every piece of information from earthquake stations and sonic sleds towed behind ships and from oil drillers' seismographs and from earth tomographies collected over a ninety-five-year period. Then we combined it with satellite data measuring the heights of the ocean surface, reverse-albedo, gravity fields, geo-magnetics, and atmospheric gases. The methods have all been used before, but never all in combination. Here's the result, a series of delaminated views of the Pacific region, layer by layer.'
'Now we're getting somewhere,' one of the scientists grunted. Ali felt it herself. This was big.
'You've seen seafloor topographies before,' the cartographer said. 'But the scale was, at best, one to twenty-nine million. What our department has produced for Level 2 is almost equivalent to walking on the ocean bottom. One to sixteen.'
He tapped a button on his palm mouse, and the image magnified. Ali felt herself shrinking like Alice in Wonderland. A colored dot in the mid-Pacific soared and became a towering volcano.
'This is the Isakov Seamount, east of Japan. Depth 1,698 fathoms. A fathom, as you know, equals six feet. We use fathoms for depth readings, feet for elevations. You'll be using both. Fathoms for your position relative to sea level, and feet to measure the heights of cave ceilings and other subterranean features. Just remember to convert to fathoms when you're down there.'
Down there? thought Ali. Aren't we already?
The cartographer moved his mouse. Ali felt flung between canyon walls. Then the image threw them onto a plain of flattened sediment. They sped across it. 'Ahead lies the Challenger Deep, part of the Mariana Trench.'
Suddenly they were plunging off the plain into a vertical chasm. They fell. 'Five thousand nine hundred seventy-one fathoms,' he said. 'That's 35,827 feet. Six-point-eight miles deep. The deepest known point on earth. Until now.'
The image flickered again. A simple drawing showed a cross-section of the earth's crust. 'Beneath the continents, the abyssal cavities are not exceptionally deep. They mostly exploit surficial limestone, which is readily eroded by water into such traditional features as sinkholes and caves. These have been the focus of public attention lately because they're close to home, underneath cities and suburbs. At last count, the combined military estimate of continental tunnels ran to 463,000 linear miles, with an average depth of only three hundred fathoms.
'Where you're going is considerably deeper. Beneath the ocean crust, we're dealing with a whole different rock from limestone, much newer in geological terms than the continental rock. Until a few years ago, it was presumed that the interior of ocean rock was nonporous and much too hot and pressurized to sustain life. Now we know better.
'The abyss beneath the Pacific is basalt, which gets attacked every few hundred thousand years by huge plumes of hydrogen-sulfide brine, or sulfuric acid, which snake up from deeper layers. This acid brine eats through the basalt like worms through an apple. We now believe there may be as many as six million miles of naturally occurring cavities in the rock beneath the Pacific, at an average depth of
6,100 fathoms. That's 36,600 feet below sea level, or six-point-nine miles.'
'Six million miles?' someone said.
'Correct,' said the cartographer. 'Very little of that is passable for human beings, naturally. But what is passable is more than enough. Indeed, what is passable seems to have been in use for thousands of years.'
Hadals, thought Ali, and heard the stillness all around her.
The screen filled with gray, shot through with squiggles and holes. The overall effect was of worms burrowing through a block of mud, surfacing and diving into the nether zone.
'The Pacific floor covers roughly 64,186,000 square miles. As you can see, it's riddled with these cavities, hundreds and thousands of miles of them. From Level 15, roughly four miles down, the density of rock and our limited technology drop our scale to 1:120,000. But we've still managed to count some eighteen thousand significant subterranean branches.
'They seem to dead-end or circle on themselves and go nowhere. All except one. We think this particular tunnel was carved by an acid plume relatively recently, less than a hundred thousand years ago, just moments in geological time. It appears to have welled up from beneath the Mariana Trench system, then corkscrewed east into younger and younger basalt. This tunnel goes from Point A – where we sit this morning – all the way across to Point B.' He walked from east to west across the front of the screen, pulling his pencil point across the entire Pacific territory. 'Point B lies at point-seven degrees north by 145.23 degrees east, just this side of the Mariana Trench system. There it dips deeper, beneath the Trench.
'Where it goes, we're not quite sure. It probably links with the Carolinian system west of the Philippines. A profusion of tunnels shoots throughout the Asian plate
systems, giving access to the basements of Australia, the Indonesian archipelago, China, and so on. You name it, there are doorways to the surface everywhere. We believe these connect with the sub-Pacific network here at Point B, but our scan is still in progress. It's a cartographic missing link for the moment, as the source of the Nile once was. But not for long. In less than a year, you are going to tell me where it leads.' It took Ali and the others a minute to catch up.
'You're sending us out there?' someone gasped.
Ali was staggered. She couldn't begin to grasp the enormity of the endeavor. Nothing January or Thomas had told her was preparation for this. She heard people breathing hard all around her. What could this mean, she wondered, a journey so audacious? Why send them all the way across to Asia? It was a stratagem of some sort, a geopolitical chess move. It reminded her less of Lewis and Clark's traverse than of the great expeditions of discovery once launched by Spain and England and Portugal.
It struck her. Their journey was meant to be a declaration, a pronunciamento. Wherever the expedition went, Helios would be asserting its domain. And the cartographer had just told them where they were going, beneath the Equator, from South America all the way to China.