Elefanten is an atomic bomb safe command center and former civil defense facility. It was decommissioned in 1998, but from it's opening in 1977 the facility was preserved exactly as it was, with temperature control and  filtered air, which made it an amazing time capsule. Armed Forces bought the bunker in 2017 and put it back into service.

The elephant is trapped in a rock room and was built between 1972 and 1977 with room for 200 people. The 3,300 m² facility was intended to serve as a command center and protection room for the City of Stockholm and Sollentuna municipal authorities in the event of nuclear war. The elephant was taken out of service in 1998 but still today is completely intact and completely decorated.

South of Stockholm there is a similar facility, the Vargen in Huddinge municipality, which has been turned into a data center.

It is possible to visit Elefanten on rare occasions, via Stockholms Länsmuseet.

Read more (Swedish):

https://sv.wikipedia.org/wiki/Elefanten,_Sollentuna