While many lava tube passages tend to be sinuous passages, the cross-sections of lava tube passages vary from tall and narrow, to classic tube shape, to keyhole-shaped. Passages tend to be tubular or oval-shaped when they initially form, as in the top photo. As lava continues to flow through a passage over time, it tends to melt and erode down the floor and make the passage taller, as in the middle pair of photos.Passages can also be stacked, with new passages forming over older ones as underground channels roof over, as shown in the bottom photo. In some tubes as many as five distinct, stacked levels are known.