Oxygen, lots of swamp gas
Standing water everywhere
1 or 2
- "Built this planet up from nothing I have. When started here I did, all there was was swamp. Other Jedi exiles said, daft I was to live on a swamp planet. Built my hut here anyway I did, just to show 'em. Into the swamp it sank. So a second one did I build. Into the swamp that sank, too. So a third one I built... which burned down and fell over and then sank into the swamp. But stayed up the fourth one did!"
- ―Yoda, on Dagobah
a slimy mudhole unfit for habitation of any kind. (Editorial note: Mudhole? Slimy? Biased, this article is.) Okay, it was a swamp planet, and the chemical composition of its bogwater was 2% water, 98% slime. It was home to dragonsnakes, giant swamp slugs, giant leeches, snails, mosquitoes, and anacondas. The least disgusting form of life on Dagobah was lawyers. The planet teemed with life, and the air was filled with swamp noises: insects chirping, slugs squishing, birds crowing, and disturbing banjo music playing.
It was also the home to Yoda and Swamp Thing. They both advised strangers, "Do not bring your evil here." There was also an evil dark side cave on Dagobah... well, actually, it started out as a pretty nice Force cave, but come on, if you were living in a muddy swamp for millennia, you'd get pretty nasty, too.