The interpretation of the name [Underdal] is uncertain, it was conjectured that a river named Unna or woman's name Unnr has influenced the first part of the name.
We find the farm already named by 1490 when Sveinung Rolvson sold Olvar Nilson 4 markebol at Underdal in Mo at Skatland. Orm Kolbeinson and Kjetil Ormson were witnesses. The debt on the farm in 1647 was substantially half of Underdal, and equaled all the farmer's property of 1 tn 2 mK. Moland vicarage owned the rest, 1 tnK.
The first skattelistene [tax rolls] do not mention Underdal, probably the farm lay deserted or was in the midst of being cleared. The first user was named Dyre. In a lawsuit at Tveiten under Midjås in 1670 some old documents were shown. In an 1589 exchange Luttan Knutson traded away 8 markebol at Hegeland in Vrådal to Tarjei Gunnuvson Midjås for 4 markebol at Underdal and all of Tveiten. Luttan got also 20 rd. This exchange of land was probably nullified, for after a judgement in 1615 Tarjei Gunnuvson's own heirs could not keep «Luttan sit Godtz fri» [Luttan comfortably free]. In a deposition letter from 1622 Dyre is mentioned. The clergyman Hr.Jens Kristenson had said to Luttan that he did not want to have Dyre at Underdal. Luttan replied to that «lad hann: bliffue der, ieg wil Aaucke Landskylden med dig» [let him: be there, I will Aaucke? land rent with you]. Kjetil Bilstad an Tarjei Midjås defended Dyre and Luttan promised to give him a choice between 1 pound butter or a buck in annual land rent. This lawsuit must have been around or before 1600 for Dyre later vanished completely from the skattelistene. In 1604 we find an Ingerid «Vedal» [Vrådal] who was probably his [Dyre's] widow.