The Underground Booty Squad
Duluth while it may not have too many tunnels, caves or other such underground exploration opportunities, it does have abandoned structures that are cool to see and explore.
Nopeming Sanatorium opened on May 22nd 1912, to battle the ongoing Tuberculosis Pandemic of that time. But as time went on the use and need for it became lower and lower. By 1971 it became a nursing home and stayed that way until 2002 when it finally closed due to funding issues and operating costs. Now it still stands unowned, with many explorers visiting every so often.
lakesuperiorstreams.org- has a map of streams and drains.
Zenithcity.com This website is dedicated to duluth, this link in particular has info on drains and pictures and stuff too.
substreet.org This website no longer exists but the archives do, this is a newer one from 2015 relating to drains. Here is an older one from 2015 with possibly different information. Here Is a general MN link from substreet.org that could be useful.