This opening, in fact, for the cable used for an anchor. Titanic had not two, but three anchors at the bow, two smaller ones and one large one. The two smaller ones were, as you can see, stored on either side of the bow, but the large one was stored in a special well on the bow. It can be seen, still in place, on the wreck today. The anchor was deployed over the bow, using a crane.

One level down from the main deck, on C deck, was a cable locker. This contained a reel of 3in cable for use with this anchor. When the anchor was to be deployed, the cable would be passed up to the main deck, and attached to the anchor. The anchor and cable could then be lowered. When the anchor was hoisted, the process was reversed. This was a long, difficult process, and as such, it should not be surprising that it was rarely done on Titanic and her sister ships. The central anchor was there not because the ship required three anchors to hold her, but because British regulations required the ship to carry three main anchors.