In 1805, Williamson puchased a property called 'The Long Broom Field' in the Edge Hill area of the city. The house on Mason Street stood next to a property owned by a Mr Edward Mason, after whom the street was named. Williamson then built several houses in the street, employing men returning from the Napoleonic war and others who had lost work as a direct result of the war's end. The gardens to each property led to nearby Smithdown Lane, save for a large sandstone drop of approximately 20 feet. With the houses now constructed, it is believed that Williamson continued to employ the men to build brick arches in the sandstone to create an exit to Smithdown Lane from the properties.

And so began the mystery of the tunnels. More soldiers were returning from the war and unemployment levels were high. Fortunately, for those lucky enough to work for Williamson, he now seemed to be obsessed with the building of tunnels. Whether Williamson had a genuine plan and motive for the excavations remains a mystery, as no plans were made. It is more likely that he saw it as a way to generate work for the men. Having retired and sold the tobacco firm, it was clear that he was not short of money and he was now known locally as 'The King of Edge Hill'.

Following the death of his wife, Elizabeth in 1822, Williamson paid even more attention to his tunnels and his eccentricity continued. It is alleged that he would instruct men to carry out pointless tasks, such as excavating tunnels only to block them up again.

Thirty five years of tunnelling ended abrubtly in May 1840 when Williamson died at the age of 70. Most of his fortune was spent on wages, not surprising given the length of the venture. There are rumours that some of the tunnels run as far as Liverpool City Centre.