I. ˈ ̷ ̷ ̷ ̷ˌ ̷ ̷ noun
Etymology: under (III) + drain
: a concealed drain with openings through which the water enters when the water table reaches the level of the drain
an underdrain below a filter bed
II. | ̷ ̷ ̷ ̷| ̷ ̷ transitive verb
: to drain by forming an underdrain
underdrain a meadow