Add either U or B to your mana pool. Counts as both plains and forest and is affected by spells that affect either. If a spell destroys one of these land types, this card is destroyed; if a spell alters one of these land types, the other land type is unaffected.
The original dual lands are some of the the best lands in Magic. Giving you access to blue and black, fetchable by cards that look for basic land types, and entering untapped, Underground Sea is the premier Dimir colored land in MTG.