What would happen if you fell into a hole through the Earth?

Ignoring the practical obstacles, the physics is elegant. Inside a uniform sphere the gravitational force on a falling body is proportional to its distance from the centre, which is exactly the condition for simple harmonic motion, so the fall through the planet is a pendulum swing. For a uniform-density Earth the one-way trip works out at 42 minutes and 12 seconds, independent of where the hole is drilled; using Earth's real density profile, which concentrates mass in a dense core, gives about 38 minutes. You would reach roughly 7.9 kilometres per second at the centre, arrive at the far side with exactly zero velocity, and fall back — oscillating forever in the absence of friction. Air resistance ruins it, bringing you to rest at the centre; and the real obstacles are that the interior reaches around 5,200 °C and pressures of several million atmospheres, and that the Coriolis effect would grind you against the wall of any hole not drilled pole to pole.

How confident this is

This is a reasoned extrapolation rather than an observation. The mechanism is understood, but the scale of the consequences depends on assumptions that could reasonably be argued differently.

Physics

• What would happen if an asteroid hit the Earth? It depends enormously on size — most burn up, a 20-metre one broke windows across a city, and a 10-kilometre one ended the dinosaurs.
• What would happen if gravity disappeared? The Earth itself would come apart — gravity is the only thing holding a planet together.
• What would happen if the Sun became a black hole? Earth's orbit would not change at all — the mass would be the same. It would simply go dark and cold.
• What would happen if you fell into a black hole? Into a small one, tidal forces would stretch you apart before you arrived; into a supermassive one, you would cross the horizon without noticing.
• What would happen if you travelled at the speed of light? You cannot — accelerating any mass to light speed requires infinite energy. Approaching it, time dilates dramatically.
• What would happen if gravity doubled? You would weigh twice as much, most tall structures would need rebuilding, and no bird larger than a sparrow could fly.
• What would happen if you were shrunk to the size of an ant? The square-cube law would break you: you would freeze, suffocate, and be unable to drink from a surface of water.
• What would happen if the speed of light were slower? Chemistry, nuclear fusion and the mass-energy relationship would all change — stars would not work as they do.
• What would happen if a gamma-ray burst hit the Earth? A nearby one aimed at us would strip a large fraction of the ozone layer within seconds, with the damage arriving as ultraviolet over the following years.