1. A tubular passage (cave) that is formed by underground streams following gently tilted beddingplane partings or fractures and are eroding channels downward through the rock. Their ceiling heights are greater than their widths. They are similar to surface canyons, but they possess roofs and are generally the same distance apart at the top as they are at the bottom. In Mammoth Cave, most are narrow and winding and may achieve dimensions of 50 feet wide by 100 feet high. If a canyon passage begins forming on an old tube passage, then a keyhole passage may result . 2. Also known as vadose canyons, these are cave passages, most commonly formed by continued floor entrenchment or incision, by a free flowing vadose stream. The passage width at any particular level is determined by the flow of the formative stream, the rate of its downcutting and the effects of any subsequent collapse. Canyon height reflects the stream's downcutting history. It depends upon the vertical distance available for erosional descent to the local base level and the time that erosional downcutting has been active, as well as upon the more obvious but less important influences of flow rate and erosional capacity. Vadose canyons commonly twist and meander sharply, while maintaining roughly parallel vertical sides. In contrast to some meanders in surface streams, underground meanders must generally be imprinted on a bedding plane before entrenchment of the canyon begins. Narrow canyon passages, commonly less than 1 m wide and more than 20 m high, are a particular feature of deep alpine caves. Perhaps the largest canyon passage in the world is that in Skocjanske Jama, Slovenia, which is over 100 m high and 50 m wide . See paragenetic cave. See also keyhole passage; passage; tubular passage; vertical shafts.