Drill, Baby, Drill
The Earth's mantle drives tectonic plates, powers earthquakes and fuels volcanoes, but most of what we know about it remains largely hypothetical. The $1 billion Integrated Ocean Drilling Program (IODP) hopes to change that. In a technological feat some compare to a moonshot, the IODP plans to drill a hole 30 centimeters (11.8 inches) wide and 6 kilometers (3.7 miles) deep through a thin part of the ocean's crust, then retrieve a sample of the mantle's slowly deforming rock [source: Levitt].
Now that we've dived into an idealized ball of rock, let's complicate things a bit with reality.
Under pressure: Boring a tunnel through the Earth would require overcoming the mind-boggling pressure exerted by 6.6 sextillion tons of rock pressing inward – that amounts to roughly 3 million times sea level pressure [sources: Locke; Plait; UCSB].
You're gonna carry that weight: A tunnel 25 feet, or 7.6 meters across (slightly smaller than the Chunnel) would displace 20 billion cubic feet (578 million cubic meters) of rock. That's a lot of rock.
It's getting hot in here: The Earth's interior is staggeringly hot due to a number of factors, including kinetic energy from formative impacts, gravitational compression forces, internal friction and radioactive decay [source: Plait]. In fact, the crust alone is hot enough to defeat current tunneling tech: The deepest hole ever dug, the Kola Superdeep Borehole in Russia, reached 40,230 feet (12,262 meters) -- only a fraction of the way through the crust -- before succumbing to high temperatures. Scientists have bored holes in the ocean floor that reach closer to the mantle, however [sources: Fisher; Levitt; Santoski; UCSB].
Mass effect: Crustal mass variations caused by mountains and sea trenches pale beside the differential densities of Earth's interior layers, which grow denser as you head coreward. Consequently, your acceleration would vary more than we described [sources: Reich; Singh; UCSB].
Fatal attraction: Due to the Coriolis effect and angular momentum, your sideways motion will carry you into a wall before you get terribly far down the shaft.
To understand why, consider a hole drilled at the equator. Whether you stand on Earth's surface or near its core, you complete one revolution every 24 hours, but you don't travel the same distance: at the surface, you travel 24,900 miles (40,000 kilometers), while, halfway to the core, you journey half that distance. You would retain that 1,000 mph (1,600 kph) eastward motion as you fell, while the walls around you would move at an ever-slower eastward rate, causing you to run into them.
To save yourself some rock rash, you could drill from pole to pole, where Coriolis has no effect. However, solar and lunar gravity, which also perturb orbiting satellites, would eventually pull you into the tunnel wall anyway [source: Darling].
Strike a chord: Fun fact: A straight line from any point to any other point through the planet would take the same amount of time to fall through as a tunnel through Earth's center. Although the tunnel would be shorter, gravity would exert less acceleration and the trip would take longer [sources: Plait; Shegelski].
On the plus side, if you wanted to turn the journey into a tourist attraction or a really long subway, the fuel cost would be negligible.