German Underground Hospital - Hauptverbandplatz
The German Underground Hospital was built by forced workers from all across Europe. They took three and a half years to excavate the area and construct the site. It's now the Channel Islands' largest structure from the German Occupation. During the war, it was mostly used to store ammunition, until 1944 when it was used to treat injured German troops after D-Day. 
The maze of tunnels covers almost 7000 squared metres and was able to hold five hundred patients - with room for more in event of an emergency. There was an x-ray room, cinema, mortuary, staff accommodation, kitchen and several emergency exit shafts.
The hospital re-opened to the public on Friday 20th August 2021 and can be visited for £4. It's well worth a trip if you're on the island.