What if the Earth were Hollow?

Digging a tunnel straight through Earth and dropping into it would turn gravity into a near clockwork ride: ignoring air resistance and rotation effects, the trip from one side to the other would take about 37 minutes, with the traveler accelerating for the first half, reaching a weightless midpoint, then slowing symmetrically on the way out. The key complication is Earth’s rotation. At the equator, the ground is moving eastward at roughly 1,670 km/h; deeper down, the material beneath still completes a rotation once per day but covers less distance, so it moves more slowly. A falling object would therefore move east faster than the surrounding rock after only a few kilometers, colliding with the tunnel wall—an effect demonstrated in spirit by miners who dropped cannon balls down a mile-long shaft near Lake Superior and found they never reached the bottom.

If the tunnel instead runs pole-to-pole, the Coriolis effect largely disappears, letting gravity dominate. For the first ~3,000 km—about halfway to the center—gravity stays close to constant because most of Earth’s mass lies below the traveler in a way that keeps the net pull similar to what’s felt at the surface. That near-constant acceleration would build speed rapidly: the traveler would be falling about 8 km every second, and the halfway point would arrive in roughly 13 minutes. Near the outer core, gravity becomes strongest, but only slightly stronger than surface gravity; as the traveler moves closer to the center, mass above begins to cancel mass below, weakening the net force.

At the exact center, gravitational pull cancels in every direction. The traveler would feel weightless—no preferred “up” or “down”—while still moving extremely fast, around 22,000 miles per hour (about 6 miles per second). After passing the center, the motion reverses: the traveler is pulled weakly at first, then more strongly, until coming to rest at the opposite side. The total time comes out to about 37 minutes.

The transcript then pivots to a different “hollow Earth” thought experiment. Earth’s real interior is hot and molten near the middle, and the deepest human drilling—the Kola Superdeep borehole in Russia—reached only 12 km before hitting extreme temperatures around 180°C. If Earth were hollow but kept the same total mass, concentrating that mass in a thin shell would remove the magnetic field, since Earth’s magnetic field is tied to the molten iron core. Without that shielding, solar wind and storms would deliver radiation more directly, making auroras visible everywhere. Inside the hollow cavity, gravity from the spherical shell would cancel perfectly, leaving occupants floating freely—though they’d need a space suit because there wouldn’t be enough air to fill the entire interior. Finally, the discussion notes that outside such a hollow Earth, gravity and orbital behavior would look essentially unchanged: falling objects would still accelerate at about 9.8 m/s², trajectories like a baseball’s would match, and the Moon’s orbit would remain the same.