Obstacle 1

Earth's Crust

High or low, wherever you go on the planet's surface, you're traveling on or above the Earth's crust. Our planet's outer layer began taking shape 4.5 billion years ago atop the fireball of asteroids, comets, and other space debris that clumped into a gooey lump to form Earth. As the surface cooled and hardened, Earth's crust was formed. It comes in two types:

When you reach down and feel the ground, you're touching continental crust. It ranges from 6 miles (10 km) up to 47 miles (75 km) deep under Mount Everest, Earth's tallest mountain. Continental crust consists of less dense and much older rock than oceanic crust.

Oceanic crust is about 4 miles (7 km) thick at the bottom of the deepest ocean trenches. It's still taking shape in these mid-ocean ridges, where molten rock erupts from cracks in the ocean floor and cools to form new crust.

Neither crust seems particularly thick. right? Tell that to the geologists and mining companies that tried digging through it. Despite using mid-ocean ridges as a starting point for digging operations, they haven't been able to pierce the crust. It's too tough, fiercely hot, and full of hazards, from pockets of molten rock to lakes of boiling sulfur.

Obstacle 2

The Mantle

Even if you manage to dig through the Earth's crust, you've literally only scratched the surface of the planet. Below lies the mantle 2, a layer of semi-molten metals such as iron, magnesium, and aluminum. The heat and pressure here are intense enough to compress carbon into diamonds, the hardest natural material on Earth. The easiest way to reach the mantle is to ride the oceanic crust. (It sinks slowly to the mantle in a process called subduction.) Once it reaches the mantle, oceanic crust melts and returns to the surface as magma in mid-ocean ridges, where it's recycled into new crust. The whole process takes about 200 million years, so you might want to pack a toothbrush.

Obstacle 3

The Outer and Inner Cores

A spherical Mars-size sea of molten iron and nickel swirls 1,800 miles (2,900 km) beneath your feet. It flows around an inner core 3 of iron two-thirds the size of the moon. (This flow of liquid iron around the solid inner core is what creates the Earth's magnetic field.) Tempera- tures in the inner core exceed 10,000 degrees Fahrenheit (5,600 degrees Celsius)-hotter than the surface of the sun-yet the intense pressure here locks the molten iron into a solid sphere. Good luck digging through that.

Obstacle 4

The Big Squeeze

The deeper you dig into Earth, the deeper your troubles. Gravity pulls trillions and trillions of tons of rocks and metals toward the planet's center, and the weight of all that rough stuff above your head increases as you dig. The pressure in the inner core is 3.5 million times the air pressure you feel on the planet's surface. Your body would suffer serious damage once you hit 27 times the surface pressure.

Obstacle 5

Tug-of-War

If not for the crushing pressure it creates around you, gravity would be your best buddy during the long haul to the center of the Earth. It's all downhill to the Earth's inner core, after all, and you'd actually experience zero gravity at the exact center of the planet (the vast mass of all that molten metal and rock around you pulls you in all directions and cancels gravity's effect). But you've only made it to the halfway point. You must repeat all the backbreaking, physics-defying work that got you here except now gravity is working against you. It's time to begin the uphill battle to dig your exit tunnel through the opposite half of the planet. That's roughly 3.958 miles (6.370 km) of molten metal and solid rock you'll need to dig through, all of it bouncing off your head as you climb up, up, and up toward the planet's surface.

Obstacle 6

Geography

More than 70 percent of the Earth's surface is covered with water, which means you're much more likely to strike seawater than sunlight when you finally reach the other side of the planet. Try to dig a hole to China from the United States and you'd end up all wet under the Indian Ocean. If you're determined to get to the bottom of things through an 8,000-mile (12,875-km) in the ground, stick to these start-and-end spots on opposite ends of the globe.