The Narrows, is the only passage from the Atlantic Ocean to St. John's Harbour, Newfoundland, bordered north and south by steep rock walls. A skilled captain is required to navigate large ships through the Narrows, known as “threading the eye of the needle”. The channel has a least depth of 11 metres and at its narrowest point near Chain Rock is 61 metres wide. From at least circa 1830, metal rings were set into the rock on either side of the Narrows so that boats could "warp in" and be hauled along to clear the cliffs and dangerous rocks in the water. In 1855, work began to blast away the submerged top of the Merlin Rock to make a safer passage for steam ships.