Subnautica/YMMV


 * Demonic Spiders:
 * Reaper Leviathans are some of the biggest threats (and sources of Nightmare Fuel) that you'll run into. The Sea Dragon and Ghost Leviathans are just as dangerous, but are easier to prepare for thanks to being rarer and having bioluminescence that allows you to keep an eye on them from a safe distance. The Reapers however have no bioluminescence, and you'll often have no idea that you're in one's sights until it grabs you from behind and roars in your face. They can kill you in two hits and will chase you down over huge distances before giving up the hunt.
 * Until you, the Warpers are another major threat. They attack in groups, can two-hit kill you with their powerful attacks, and (true to their name) can forcibly warp you outside of the safety of your underwater vehicles so they can either attack you themselves or put you in biting distance of other dangerous sea creatures.
 * Goddamned Bats: To those who don't consider them to be Demonic Spiders, Warpers are this due to their tendency to teleport you outside of the safety of any vehicles you're riding. Even if you're good at dodging their attacks, they're still annoying to deal with.
 * Special Effect Failure:
 * Other than for yourself, anything you can pick up and carry, and any vehicles you build, there is virtually no collision detection in the game. It is possible to build a base entirely around a moving object, and have it move through your base like a ghost.
 * The leviathan(s) at play near the engines of the crashed Aurora have no collision detection or AI, so as long as you're above water, they're effectively simple animations. If you build a series of foundations at sea level in the area, they will surface ghostlike through them and ignore you -- even if you're standing on top of them when they do.