I knew that the Naval Base at Farragut , Idaho, on Lake Pend Oreille has been testing submarines for many years, but I was just reading that the base has been there since the Second World War, and is actually one of the top secret naval bases where new submarines are tested out. The lake is one of the deepest lakes in the United States, and because the bottom is also similar to the ocean, they are able to simulate conditions that the subs would be operating in. Since the base is totally in the UNited States, we can test without worrying about other countries spying like could happen if we ran the tests in the open waters in the ocean. This article shows some of the submarines being tested, and is pretty interesting history about the Farragut Naval Base. https://foxtrotalpha.jalopnik.com/the-navys-most-vital-and-secretive-submarine-base-is-in-1590794426