Would bet that the old Navy sub base at Water Island, which is honeycombed with tunnels also has underwater chambers that a small sub could fit into.
https://en.wikipedia.org/wiki/Water_Island,_U.S._Virgin_Islands
The main attractions are beaches, including Honeymoon Beach, plantation ruins, Fort Segarra, an underground fort partially constructed by the U.S. during World War II, and scuba diving site Supermarket Reef in Limestone Bay.
While the rest of the Danish West Indies were purchased by the United States in 1917, Water Island was not purchased by the U.S. until June 19, 1944, when it was purchased for $10,000 to protect the submarine base on Saint Thomas during World War II.[2]
From 1944 to 1950, the island was under the operation of the Department of Defense. Construction on Fort Segarra commenced in 1944 but the fort was abandoned incomplete in 1948. The United States Army's Chemical Warfare Division used sections of Water Island to test chemical warfare agents, including predecessors of Agent Orange, from 1948 until 1950. It was then turned over to the Department of the Interior and leased out, primarily to residential tenants.
A coincidence that this came out yesterday?
https://www.dailymail.co.uk/news/article-8645191/Satellite-images-capture-single-Chinese-nuclear-submarine-entering-Bond-lair-cave-complex.html
Satellite images capture single Chinese nuclear submarine entering 'Bond lair' cave complex - but all other berths are empty sparking fears the subs are all shadowing the US fleet
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New photos show the Yulin Naval Base on Hainan Island in China - a vital strategic site for Beijing
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For the first time, a submarine is seen entering an underwater cave network constructed below a mountain
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The 'James Bond lair' was built around 12 years ago, experts believe, and is one of several managed by China
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Submarines have been seen at the berths before, but never entering the caves, experts believe