A mysterious network of tunnels and chambers is gradually being uncovered below the city of Liverpool, England.

A group of 20 volunteers have been working to uncover the tunnel's secrets for the last two decades and they have since become a major tourist attraction.

The tunnels were built in the 1800s by philanthropist and tobacco merchant Joseph Williamson - but nobody knows why.

Some have speculated that they may have been most commonly used for Mr Williamson to quarry stone, creating work for unskilled labourers.

But others disagree, saying they were constructed so that the merchant could smuggle in goods. There's even a theory that he had them built because his wife was convinced the world was about to end.

According to history, for more than 36 years Mr Williamson had workers build the chambers and tunnels, costing him millions in wages for the labourers.

Volunteers work every Wednesday and Sunday, and have removed thousands of tonnes of rubble and rubbish in their two decades.

One chamber, which is 12 metres high, took the group four years to clear out.

Newshub.