From wikipedia: "The naval harbor is estimated to be 60ft high, built into hillsides around a military base. The caverns are capable of hiding up to 20 nuclear submarines from spy satellites. The harbor houses nuclear ballistic missile submarines and is large enough to accommodate aircraft carriers."
Covert, Military - Navy and Coast Guard
I call BS on the last claim. The entrance is only 75 feet wide. That is "a bit narrow" for an aircraft carrier.