Flooded drain-type tunnels underneath the Twin Cities milling districts. These tunnels were the outfalls from mill waterwheels, which took water from the Mississippi River to power their systems in the days before electricity. Today most of the tailraces are sealed up, gated off, or full of raging water, but a few are still accessible.
A walkway atop the central water channel in one of the tailraces. Side
tunnels led off to other flooded tailraces. The cieling was about 7 feet
up, water was about 5 or 6 feet down, and of an unknown depth (more than
5 feet).
At the end of a tailrace, up inside the guts of a mill. This particular
mill has been refurbished above, leaving a sealed crawlspace with an original
floor, new cieling, and lots of odd machinery and junk trapped in between.
There are also a lot of rats, the only (libe) ones I saw in or under the
Twin Cities. The last photo poorly illustrates the really dodgy shaft down
from the mill basement into the tailraces, a several-floor drop through
rusty gears, pipes, cables, and other random machinery. If you look closely
you might make out some stairs way down there.