It was in the early stages of the Second World War, when the paint was barely dry on the five-year-old tunnel, that it caught the attention of those responsible for planning infrastructure in the wartime economy. Top secret correspondence was sent from the wartime ministries of Supply and War to the Ministry of Transport and the Liverpool City Engineer.
There was clearly potential in two miles of underground tunnel, mostly 13.5m (44ft) in diameter, though there was little consensus on what exactly they might like to do with it.
All the same, one William Chamberlain from the MoT convened with the City Engineer and sent back a (top secret) list of possible ways in which it could be commandeered. He was very fast to point out that Liverpool was the busiest port in the empire, and that most of its substantial cross-river traffic went through the tunnel, many of the old steamer ferries having been decommissioned shortly after the new road opened.
The possibilities therefore looked at how to keep it partially open, perhaps only permitting goods vehicles necessary to the war effort. One idea was to partition it vertically, leaving two traffic lanes open and using the other two for storage, giving a space more than two miles long and 22 feet wide. The more realistic proposal was to close the narrower branch tunnels to New Quay and Rendel Street, or to close the main lines between the branch junctions and the surface, leaving the narrow branches as the only ways in and out.
The principal requirement was somewhere to store aircraft. In 1940, American aircraft were being transported to Britain by sea and unloaded in Liverpool, and it was anticipated that a surplus could eventually be built up. The planes were to be dismantled and discreetly stored underground somewhere.