The theory on the cellulose was that, if penetrated by a projectile at this location, the cellulose, when brought into contact with the inward flow of water, would expand. The expanding cellulose would squeeze off the water flow, and basically seal the hole. The idea sounded good in theory, but did not work in practice. Even normal leakage caused the cellulose to rot and smell. The material was soon removed.
 

The cofferdam can most plainly be seen when traversing the wing passage, which is a long passage that passes between the outer hull and the coal bunkers, etc. just at the waterline.

The wing passage on OLYMPIA is somewhat reminiscent of the wing passage on the USS MAINE. On the MAINE the wing passage became well known, since men moving along this space would have had to pass by and touch the bulkhead enclosing the coal bunker, thought by some to have been on fire. Crewmen who passed through this area on the MAINE noticed no heat coming through the bulkhead, one of the arguments against the coal bunker fire theory of her loss.