Boo Woods

Boo Woods is a location in the Mushroom World and is first featured in Luigi's Mansion. Boo Woods is extremely large and full of many dead trees; E. Gadd's lab is found here along with the mansion Luigi won. Boo Woods also appears in Mario Kart DS and Mario Kart 7; in these games it has a large swamp and some of the trees can move around. As the name implies, Boos haunt this area along with the leader of this area, King Boo.

Background
Boo Woods is first named when Young E. Gadd mentioned it in Mario & Luigi: Partners in Time. After his lab was destroyed by the volcano of Thwomp Mountain, he says he's got "a hankering to do some paranormal research" and that he plans to move to "an affordable lab at the edge of Boo Woods".

Luigi Mansion
Prior the events of Luigi's Mansion, Mario goes to Boo Woods to go to a mansion that Luigi owns, waiting for his brother to meet him there. However, he goes missing. In the beginning of the game, Luigi goes to Boo Woods to find his mansion he won in a contest he didn't even enter. In the game, Boo Woods appears more like a background than an actual location. Luigi can go outside to the courtyards of the mansion, where Boo Woods is seen. The woods can also be seen in the balcony of the mansion

Mario Kart series
Although not present in Mario Kart: Double Dash!!, Boo Woods make appearances in Mario Kart DS and Mario Kart 7.

Mario Kart DS
In Mario Kart DS, outside the mansion is Boo Woods, which has a graveyard surrounding the mansion and beyond it, a swamp of mud with strange walking trees. When a character runs into one of these trees, instead of spinning out or rolling over, the character instead will just get stuck on the tree as if it were a wall.

Mario Kart 7
Boo Woods also appears in Mario Kart 7, along with the retro course of Mario Kart DS, Luigi's Mansion.

Trivia

 * Boo Woods is extremely similar looking to Forever Forest, which may mean that Boos prefer dark, spooky forests to other places, as they live there also.