[2] Within two years, cost-saving measures pushed back its completion date and progress was slowed by water levels much greater than had been expected.

It proved difficult to secure skilled help, some tenders went unanswered and Benjamin Outram withdrew from the venture.

[4] Benjamin Outram was appointed the consulting engineer after his report in October 1793 estimated the cost of the canal and tunnel at £178,478.

Outram thought that geology of the hill through which the tunnel would pass was of gritstone and shale, and would not present any difficulties.

Considerable effort had been spent constructing small tunnels to supply waterwheels to raise spoil and water from intermediate adits.

[4] By the autumn, concerned that such work was expensive, Outram abandoned building extra workfaces and concentrated on boring out from both ends.

[6] Other factors had slowed progress: a shortage of funding and poor working practices also contributed.

[4][7] Cutbacks in drainage provision hampered tunnelling, as larger quantities of water entered the workings.

In September 1797, Outram advised the committee that Thomas Lee, the contractor, had made large losses as a result of the difficulties and could not complete his contract.

He was awarded more money for timber, an increased rate per yard for completion and an extra year in which to finish the tunnel.

In October 1800, the Peak Forest Canal Company, who were keen to trade, suggested a tram road should be built to bypass the tunnel until it was completed, but no action was taken.

Soon thereafter, mine owner Matthew Fletcher was asked for his opinion; he suggested that time could be saved by tunnelling in both directions from Redbrook pit, which was kept dry by a large steam engine.

It recommended that a towing path should be built through the tunnel, but the extra cost and delay were not affordable.

Desperately short of money, the canal company obtained a new Act of Parliament in 1806 to raise additional finance and allow an extra toll for using the tunnel.

[11] On 26 March 1811, the tunnel was complete and a grand opening ceremony was held on 4 April; a party of invited guests, followed by several working boats, entered the tunnel at Diggle and completed the journey to Marsden in one hour and forty minutes.

A survey carried out before restoration using a modern measuring system gave the length as 5,675 yards (5,189 m), which is the accepted figure.

The canal company introduced one-way working, for which one end of the tunnel was closed by a locked chain to prevent access unless authorised.

[14] Writing in 1948, L. T. C. Rolt described the journey as taking two hours, during which the speed was kept very low to avoid damage to the boat.

[16] The canal had become obstructed in several places on both sides of the Pennines and the tunnel, which had become unsafe, was closed by large iron gates at each end.

They are level for the whole length, which had the operational benefit of providing the only section of level track on the line where water troughs could be installed to provide steam locomotives with water without requiring the train to stop.

When the economic case became clear, Thomas Nelson, who built the first railway tunnel, was awarded the contract.

Construction was disrupted by strike action by tunnellers and bricklayers over disputes about payment and shift length.

A. MacGregor and carried out by 1,800 men who lived in the paper mills at Diggle and 54 wooden huts near the eastern side.

The opening of the double-track tunnel provided additional capacity for the L&NWR, allowing them to temporarily close the single bores for maintenance.

[25] The visitor centre is about half a mile (0.8 km) west of Marsden railway station, reached via the canal towpath.