French navy decided to found submarine base at Calais, France according to the Dutch magazine Marineblad dated 1905-1906 no. 9

An item referred to the magazine Mitteilungen a.d.G.d.S. reporting that the submarine base first at Dunkirk, later at Boulogne now will be founded at Calais, which it’s an excellent place lying along the narrowest part of the Channel with on the other side the new British naval base Dover. With a small number of submarines was it possible to control the Channel and prevent a or made it very difficult to blockade of Dunkirk and Boulogne.