Your recent piece about the super sewer under the Thames left the impression that at 85m depth, this was somehow pushing the frontiers of technology.
There are many coal miners still alive, who regularly tunnelled at 300m or more under rivers and lakes, including building railways to get miners to the coal face, and the coal out. In other parts of the world, diamond mines in South Africa are over 400m deep.
Perhaps Thames Water might recruit an ex-National Coal Board mine engineer to advise on the new super sewer.
Prof. Lewis Lesley, 30 Moss Lane, Liverpool L9 8AJ