The Channel Tunnel was constructed by Tunnel Boring Machines (TBMs) starting at each side and meeting deep underneath the English Channel.
There was no way to reverse the machines, so the original plan was to turn them to the side after the tunnels had met, and leave them permanently entombed underneath the sea.
This technique is common when constructing tunnels.
However, it became apparent that the machines were in good condition, and so they were allowed to continue tunnelling after turning. The France-bound TBM, will emerge near Rotterdam in 2021, whereas the UK-bound one is now heading for Dublin, but is not expected to make landfall until 2037.
This raises the potential of creating an international railway interchange on the sea bed.