Millions of people pass under the River Thames each day, by rail, road or >foot. We take it for granted. Just two centuries ago, the idea of going
under the river would have seemed fanciful. Today, there are 20 different >ways open to the public, and at least 25 further tunnels that most of us >can't access.
Makes a good point and iron rings in wet enviroment eventually corroding
away. Someone must still go in there occasionally to make sure it doesn't collapse.