If it were being built today, a 44ft (13m) diameter tunnel under a river estuary, with two branches, underwater junctions and four tunnel portals to be squeezed into city centre locations would be an incredibly large-scale project.
In 1923, the only viable prospect was to have it excavated by hand. No tunnel of comparable diameter had been built before, and nothing existed to match its length or its complexity. No ground surveys could be carried out. No mechanism existed by which central government could plan such large-scale schemes. The railways, which held enormous political sway, were vociferous opponents, given that they operated the only existing river crossing. The odds were, really, stacked against it.
All the same, the Mersey Tunnel Joint Committee was formed in 1923 and within a little more than two years work had started. When it opened, it was the engineering marvel of the world.