Having bottlenecks also made escorting somewhat easier. Gibraltar and the English Channel were some of the most heavily fortified places in the world. Gibraltar was only passable by U-boats in relative safety early in the war; eventually they were taking 25% losses just in the crossing (not to mention that once U-boats entered the Mediterranean they were considered stuck there and unable to attempt departure due to strong west-to-east currents into the sea). By 1940/41 transiting the Channel was avoided by U-boats due to the number of mines, aircraft, and escorts present.