Fort Ogilvie Submarine Mine Observation Post was built in 1890 as a replacement for a similar site (York Redoubt Submarine Mine O.P.) on the slope to the front of York Redoubt. The observation post is accessed through a small tunnel and has a communications area on the ground level. There is a short staircase up to the observation platform which would have included a plotting table. There was an underground Test Room to the rear of the site, built into the side of Fort Ogilvie, but this appears to have been backfilled and buried.
There was a separate Submarine Mine Observation Station at Ives Point below the Ives Point Battery.
in 1890 the Halifax Minefield was moved further North, close to the boom that ran from Point Pleasant to Ives Point on McNab’s Island.