Here's why:

* The crust is the outermost layer of the Earth. It's relatively thin compared to the other layers, ranging from about 3-50 km thick.

* The mantle, the layer beneath the crust, is much thicker and hotter. The extreme heat and pressure make it very difficult to drill through.

* The core, the innermost layer, is even hotter and denser. It's impossible to drill through with current technology.

While some drill holes have reached depths of over 12 km, all are still within the Earth's crust.