The Headquarters of Western Approaches Command, from whence THE BATTLE OF THE ATLANTIC was fought.
It was below the streets of Liverpool, and amongst other things included the giant vertical plotting board. See "Western Approaches War Room and Ships."
On it were shown all the convoys going both ways to and from USA to UK, and all the information from radio intercepts, and the reading of the German Navy codes, about the location and plans for the U-Boats at sea, anywhere in the world, but in the main, in the North Atlantic.
It was the nerve centre for the BATTLE OF THE ATLANTIC.
There was a direct telephone line to Winston Churchill in London.
I was able to visit these reconstructed HQ in Liverpool, when I went to take part in the 50th. Anniversary celebrations of winning the Battle against the U-Boats in 1993. May of 1943 being the date when we won that fight, that is why ONS 5, and the sinking of 7 boats became a defining point in that victory.
The plaque denotes the importance of these Western Approaches Head Quarters, if the U-Boats had not been defeated, Britain would not have survived, there would have been no arsenal base to build for a second front, and the invasion of Europe will final victory could and would not have come about.
Which is why Churchill made his famous remark. "The only thing that really frightened me in WW2 was the Battle of the Atlantic." and of course the inference that we may lose that Battle.