On our visit to Vladivostok in Siberia we got to tour a military fort, the 7th Fort, one of 16 fortresses guarding the city. The forts are all underground and the one we visited had the distinction of being the only fort with a Kitchen and an Indoor bathroom.
The fort was built between 1910 and 1916 and was the last military project of Czar Nicholas. It is supposedly able to withstand a Nuclear attack because the walls are so thick.
Not sure you can make them out, but there was a troup of Russian boy scouts touring as well that day.
Inside a Russian undeground Fort in Sibera There seemed to be endless tunnels all over everywhere...
The fort did have an interesting if small museum, but my camera had run out of juice and I didn't have any darned batteries... oh well.
Now you don't have to see the Fort when you visit Vladivostok... you've seen it... try a different tour.