Relatively little is known about the interior of the Earth, since only the surface, called the crust, is accessible to us, extending to a depth of 5 to 60 kilometers. The Soviets once attempted to drill down to the next layer, called the mantle, but gave up after 12 kilometers. Any "terranaut vehicle" attempting to reach the core of the planet would have to chew through some very substantial rock extending to 3,000 kilometers in depth which gets progressively denser and hotter. The core is presumed to be composed principally of iron, with about 10 percent alloy of oxygen or sulfur or nickel, or perhaps some combination of these three elements. The outer core is liquid, while the inner core solid. It is believed that the temperature of the core is 7,000 degrees Fahrenheit and that the pressure is millions of times higher than the atmospheric pressure at Earth's surface. Small wonder that humans have been able to reach the surface of the moon first.