A mountain pass is a low point or natural defile traversing a mountain range or elevated area, providing a relatively easier path for passage through otherwise impassable terrain. Geologically, these features represent localized weaknesses in the crust that have been exploited by erosion over vast spans of time. From a technological perspective, the identification and eventual establishment of a pass are critical points in transportation infrastructure, allowing for the connection of separated regions. The engineering challenges associated with passes involve overcoming extreme grade changes, varying atmospheric conditions, and challenging underlying geology to create viable routes for human use.