During the Napoleonic Wars a safe inland route between the royal dockyards at Woolwich and Chatham seemed an eminently sensible idea. Work on what was eventually to become the Thames and Medway Canal began in 1800.
A year later the length from Gravesend on the Thames had reached Higham - where it stalled for 18 years.
A chalk ridge blocked the shortest route to the Medway.
Various diversions were examined before the decision was taken to excavate what would be the largest canal tunnel in the UK, one that could take 7m beam Thames barges.
Five years of excavation followed, with the tunnellers digging 12 shafts down from above and working several faces at the same time. Even then chalk falls were common.
The worst lengths were lined with masonry, and a pumping system installed.
Total length was 3.5km, the second longest in the UK, with an 8.5m wide waterway, 2.4m depth of water and 1.5m towpath. Shortly after it opened in 1824 it became obvious that the canal could never be economic if only one vessel could travel through the tunnel at a time. In 1830 a massive pit was dug near the halfway point and an open air passing bay was constructed, splitting the tunnel into two separate structures 60m apart.
Still it failed to thrive, and competition from the new railway companies was starting to bite. In a desperate attempt to stay in business the canal company built a single-track railway through the tunnel, with one rail on the towpath and one supported above the water. In the end they gave up and sold the ill-fated enterprise to the South Eastern Railway Company in 1845.
The canal was filled in and two tracks laid. Over the years the chalk falls continued, and more sections were lined with everything from traditional masonry to Armco culverting.
Eventually 60% was protected in this way.