I think that this has gone on long enough, so I'll give the answer, which is the Severn Tunnel.
This from the Great Western Archive:
"With economic pressures, several schemes were put forward for a crossing of the Severn as several railway companies vied with each other for the lucrative coal trade. The most popular schemes were crossings by a bridge as tunnelling was seen as a risky and unpredictable method. One scheme that was put forward was by John Fowler and Hamilton Fulton was for a high-level viaduct 2¼ miles long entitled 'The South Wales and Great Western Direct Railway'. Standing 120 feet above high-water level it was to have three openings, one of 600 feet and two of 265 feet. At a cost of £1,800,000, it was to be a formidable undertaking, however, the risks to shipping avoiding the bridge supports in addition to the mists and fogs of the estuary were hard to calculate. Nevertheless, the planning for this bridge, or 'Fowler's Line' as it became known, received parliamentary approval.
At the same time, Charles Richardson, who with 25 years experience of tunnelling which included Box and Thames Tunnels, presented a case for a tunnel under the Severn. He argued that a tunnel would carry none of the risks of a bridge and it would cost just £750,000."
The Great Spring is open to anyone who wants to drink from it .... !