How can a vertical drillhole bend to allow horizontal storage?

Directional drilling of curved drillholes is a highly developed art. The curvature is gradual; the initial hole is vertical, and at the “kickoff” depth a gradual curvature begins; the transition to horizontal has a radius of curvature that is between 200 and 1000 feet long. There are no abrupt turns. The long steel casing pipe that is inserted to line the drillhole can make the turn.