There are nine bunkers in Mystery Lake. You will find them useful since they contain a lot of necessary things you can later use. Bunkers can contain, food, medication, flares, matches, ammo,and so on. Supplies are the main factor in the game that keeps you from dying, so if there are any supplies that are so conveniently lying around you should take them. There are also some things that will prove useful like lanterns, knives, tanks for storing water, etc. that you can find inside the bunkers.
You can check out the video for detailed information about where to locate the hatches and what exactly you will find inside the bunker.