Guernsey is covered in German defences, perhaps the least prominent from above ground, yet the most impressive are the vast array of tunnels. Unfortunately most tunnels on the island are unlined and have collapsed. However Ho.40/Ho.7 are open as The German Underground Hospital, La Valette is open as a museum too.
We advise against visiting tunnels that are not open for visitors. They are some of the most dangerous remains from the occupation on the island. Sections of tunnels have collapsed, and poisonous gas has filled many.