An annoying problem for humans, who like to boast   about all the distant planets and moons we have explored, is that we've never taken a good look right under our noses. The inside of the earth is relatively close, but how can we get there? The deepest oil well enters a mere six miles into the crust   (the center of the earth is about 4,000 miles deeper). Russian scientists dug the deepest hole in Siberia, but bottomed out at about 7.5 miles below the surface. The Mohole project, a U.S. plan in the 1950s, called for drilling a hole 25 miles down to the boundary between the hard rocks of the crust and the soft mantle  . Sadly the project involved government supporting. It gets harder and harder to drill deep into the earth because rocks get softer and softer. Hard but easily broken at the surface, rocks become plastic at depth, and the pressure caused by the weight of the overlaying crust ---  about 52,800 pounds per square inch at a depth of ten miles, makes further drilling impossible. What little we know about the inside of the earth (like the fact that there's a crust, a mantle, and a core) comes from indirect evidence, such as the analysis of earthquakes. So maybe it's time for a thorough new method to explore the earth's inside. Scientist David Stevenson says we should forget about drilling holes. Instead, we should open a crack  . Stevenson suggests digging a crack about a half mile long, a yard wide, and a half mile deep (not with a shovel) but with an explosion on the scale of a nuclear bomb. Next, he'd pour a few hundred thousand tons of molten   iron into the crack, along with a robot. The iron, thicker than the surrounding crust, would move downward at about 16 feet per second, carrying the robot with it and opening the crack deeper and deeper. The iron mass would drop for about a week and 2,000 miles to the outer edge of the earth core, the robot sending out data to the surface. Stevenson compares his idea to space exploration. "We're going somewhere we haven't been before,"he says. "In all possibility, there will be surprises." This idea can probably be put in the drawer marked with Isn't Going To Happen. The robot would have to survive temperatures that would melt pretty much anything. But Stevenson's idea may inspire a new look at an old problem. Great things can come from what seems like impossible ideas. How deep have we gone into the earth until now?
6 miles. 4,000 miles. 7.5 miles. 25 miles.