But now, as she lay flat looking at the giant drilling rig, the point was that the northern hemisphere was even lower than it first appeared, for its floor of bedrock was amazingly deep, as much as five kilometers beneath the surface of the dunes. The impact had blown that deep, and then the depression had mostly refilled, with a mixture of ejecta from the big impact, windblown sand and fines, later impact material, erosional material sliding down the slope of the Great Escarpment, and water. Yes, water, finding the lowest point as it always did; the water in the annual frost hood, and the ancient aquifer outbreaks, and the outgassing from the blistered bedrock, and the lensing from the polar cap, had all eventually migrated to this deep zone, and combined to form a truly enormous underground reservoir, an ice and liquid pool that extended in a band all the way around the planet, underlying almost everything north of 60° north latitude, except, ironically, for a bedrock island on which the polar cap itself stood.

Ann herself had discovered this underground sea many years before, and by her estimates between sixty and seventy percent of all the water on Mars was down there. It was, in fact, the Oceanus Borealis that some terraformers talked about— but buried, deeply buried, and mostly frozen, and mixed with regolith and dense fines; a permafrost ocean, with some liquid down on the deepest bedrock. All locked down there for good, or so she had thought, because no matter how much heat the terraformers applied to the planet’s surface, the permafrost ocean would not thaw much faster than a meter per millennium— and even when it did melt it would remain underground, simply as a matter of gravity.

Thus the drilling rig before her. They were mining the water. Mining the liquid aquifers directly, and also melting the permafrost with explosives, probably nuclear explosives, and then collecting the melt and pumping it onto the surface. The weight of the overlying regolith would help push the water up through pipes. The weight of water on the surface would help push up more. If there were very many drilling rigs like this one, they could put a tremendous amount on the surface. Eventually they would have a shallow sea. It would refreeze and become an ice sea again for a while, but between atmospheric warming, sunlight, bacterial action, increasing winds— it would melt again, eventually. And then there would be an Oceanus Borealis. And the old Vastitas Borealis, with its world-wrapping black garnet dunes, would be sea bottom. Drowned.

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