Supply pods are supposedly what landed when the Unity ship broke apart and scattered across the planet, and thus contain things from that ship. They also sometimes contain alien lifeform, sonar scans of the surrounding area, whirlpools that send a ship to a different part of the ocean, and even monoliths that somehow fit inside them and magically appear when you open them. At your starting game settings, you can determine whether or not to have supply pods randomly scattered across the planet. The standard rules allow this.