You enter the tunnel heading north out of Oklahoma City. They wanted it in an area without seismic activity. The incline is barely noticeable at first, a grade gentler than Highway 70 coming down off the Rockies from Denver. The road you're on gently curves to the right. You're on a spiral to the center of the Earth.

Delicate reliefs of local fossils decorate the walls. All are from the Permian Era or earlier. Only the western third of Oklahoma was above water when dinosaurs ruled. What you see is early amphibians and insects, enlarged enough to be visible at seventy miles an hour. After the reliefs come a history of the oil industry, from Spindletop on.

At first the traffic is heavy. Lots of people come to drive down the first leg of the six lane highway. The Earth's crust here is only about thirty miles thick, so it's a morning drive to get down to the mantle where there's a shopping mall, a rest area, and the turnaround to ascend back to the surface.

You're not here for entertainment. Pulling into a Texaco, you fill up and head for the neon arrow pointing down. Here there are only two lanes in each direction. The cars emerging from the tunnel look worn and dusty.

There are thermometers spaced every hundred miles tracking the temperature increase as you descend. They start at 1600 degrees Fahrenheit. The rock outside is viscous, flowing sullenly under enormous pressure.

The grade here is twenty-five percent, so you're driving four miles to descend one mile. This is the long slog, seventy-two hundred miles to the outer core. Gas stations, restaurants, and motels break the monotony. Hilton opened a hotel at the halfway marker, but sold it to Motel 6 soon after the opening of the highway.

The lights here are spaced farther apart, red-shifted as the highway's architects took advantage of the surrounding radiance. They get brighter when you enter the outer core, where molten nickel and iron glow. Gravity loosens its hold as you travel deeper, the car drifts until magnetic guides grip it and carry it down, where the thermometer reads nine thousand degrees.

And here you are, at the Hub, the Earth's core. From here highways arc up to Australia, to China, and to France. But you won't ascend. You'll stay here, find a job, and live out your days. You give the car to some other penitent ready to rejoin the world above.