> energy required to drill That depends on the rock type. In london, most things are clay, so not actually that solid (ie it needs shoring up immediately, and

> energy required to drill That depends on the rock type. In london, most things are clay, so not actually that solid (ie it needs shoring up immediately, and will collapse without supports, hence the travelling shield) manhattan schist appears to be reasonably hard (not granite, but also no clay)