A cypress knee is a distinctive structure forming above the roots of cypress trees of some of the species of the subfamily Taxodioideae, such as the baldcypress. They are believed to be a type of aerial root, and are often found in swamps. Cypress knees can grow to considerable size. The largest on record, along the Cache River in northeastern Arkansas, is 3 m (9.8 ft) high and 0.9 m diameter at water level. The tallest, along the Suwannee River in North Florida, is 4.3 m (14 ft) in height. Some current hypotheses state that they might help to aerate the tree's roots, create a barrier to catch sediment and reduce erosion, assist in anchoring the tree in the soft and muddy soil, or any combination thereof.