“How did the U.S. subs get there; the Caspian Sea doesn’t connect to the open sea.”
I don’t know but there is a US Navy submarine base — with a sub — in Idaho.
It’s in lake Pend Oreille, ID.
Congressional\Senatorial earmark funding can go miraculous things!
Obviously a joint Navy/Air Force flying sub.
That sub in Lake PendOreille is a scale model for testing hull shapes. Not a real nuke. But it is possible to drive a real sub up the Columbia, then the Snake, all the way to Idaho. With a bit of work they could drive one to Boise.