Hermit crabs molt periodically, meaning they shed their exoskeleton and a new, slightly larger one hardens in it's place. Generally, healthy crabs in good crabitat (crab habitat) conditions, will molt underground. Often, hermit crabs will coordinate their efforts into a mass molt, and every inhabitant will go aground. These periods of time can extend into months, and can be boring, worrisome, or even frustrating for their human keepers. You may feel that you have "Pet Dirt."
It is important to remember to feed and water your pet dirt. Molting crabs do surface occasionally to partake. You can smooth out the top layer of sand ever-so-gently (don't create a cave-in) and look for crab tracks the next morning. This will alleviate any fears you may have that your may have passed away. Take care to keep your substrate moist during this time as well. Dry sand or coconut fiber can crumble cave in on molting crabs in their burrows. TIP: Try pouring water into a corner of the tank where you are certain no crabs are hiding, being certain to do so slowly enough not to allow the water to pool on the tank floor. The substrate should wick up the water to distribute moisture evenly. Now is not the time to clean or re-decorate the crabitat. It is important to be sure they are undisturbed because the crabs are very vulnerable during this natural process and require peace and quiet.