In 1943, the Nazis started construction of a massive concrete bunker to build submarines in. At a planned production rate of 12 subs per month it was meant to be the German war machine's most important naval project. The largest free-standing bunker in Germany – around 426m long and 97m wide, up to 27m high and with 4.5m-thick walls – it was built by up to 12,000 slave-labourers from POW and concentration camps, under intensely cruel conditions; thousands died at the site.
Allied bombing badly damaged the bunker when it was 90% completed, just a month before the Germans surrendered; not a single submarine was ever built there, and today it stands as a symbol of the futility of war. It's an eerie, ruined place, with rainwater and sunlight leaking in through the bomb craters in the ceiling; it's made all the more haunting by the stories of survivors, which you can read about and listen to in a free audioguide, available in English (ID deposit required). A 1.5km-long path runs through and around the bunker, with the audioguide and English signage providing information at 25 stops.
To get there from Bremen, take the Regio-S-Bahn train RS1 to Farge (€2.80, 44 minutes, every 30 minutes), the last stop; from the station, cross the road and walk to the right to the bus stop. Take bus 90 four stops to Rekumer Siel and follow the sign down a side street. If you go in the morning you can stop in Vegesack on the way back to visit the Schulschiff Deutschland as well.