A ship is a nothing but a set of welded steel compartments which holds everything it needs to do it job. There are a few spots located all over the ship that are too small to be useful. These are empty and full of nothing but air. These spaces are called voids. They are small corners of compartments that would other wise go unused.

Voids are normally sealed shut and forgotten. Some voids have a small opening in them so they can be checked for water. Voids do provide a useful function in construction of a ship. They provide an air pocket and buoyancy for the ship.

There are sorties of voids on various ships. Most of the stories center on aircraft carriers, since they are large. The story goes something like this. During a period of overhaul a carrier was in dry dock. Some new pipe or cable needed to be run so hole was cut in a bulkhead so a void could be accessed. When the hole is cut light can be seen. After a larger hole is cut it is a totally intact compartment that was for some reason welded shut and forgotten since the ship was built.

As far as I know these stories are not true nor have any foundation.