Tunnel Boring machines (TBMs) fascinate me. A machine that is part excavator, part factory, part structural support. And generally custom made for every job, wicked expensive, and by all accounts very high maintenance. Seemingly only "slightly" better than just digging the hole the old fashioned way with sand hogs and trucks.

Given their nature, I have always wondered whether or not it would be possible to use the excavating end as a 'portal' to manually excavate in the case of breakdown, such that you could get to the "outside" of a damaged TBM but clearly that isn't a design feature (the Swiss needed to get to theirs as well as I recall on their tunnel project through the Alps. That we got a pair under the English channel seems amazing.