Hidden in the hillside near the town of Dannes in the Pas de Calais region of France is this WW2 German underground torpedo and sea mine storage facility.
Known as strongpoint Stp Hasso, this is a massive construction which boasted its own railway line direct to the port of Boulogne-sur-Mer where the ammunition was delivered to S-Boats and Kriegsmarine ships using the large port.
There were 44 underground storage areas and two tunnels for trains planned here. Today you can see around 15 areas which all feature a dog-leg design and ventilation systems to protect the explosives stored inside. Many of the tunnels are closed off but you can access a few if you're careful, remember to take a decent torch!

On the hillside behind Stp Hasso was a large scale bunker site known as Stp Ruine which featured a mixture of gun casemates, shelters, and labyrinth of underground tunnels linking them together.
One of the most dangerous sites to visit, access to the site and tunnels has now been (thankfully) closed off and it is now illegal to go into the fields where the remaining structures are.
However, you can still see a line of R669 casemates from the farm access road which runs through the site, although these are part of the farm and used by cattle so you can't get up close.
We don't normally say this about bunker sites but some, in particular Stp Ruine, are best avoided.