A non-nuclear, airless submarine capable of 30 days underwater.
Sweden has commissioned a number of these subs which are able to sail underwater for up to 30 days using a Sterling engine.
http://en.wikipedia.org/wiki/Gotland_class_submarine
The US has borrowed one of them to practice detection. It's an impressive machine for a government which is interested in protecting it's own waters. They will make it very difficult for anyone to land on their territory.
The newer subs of the US are more geared to supporting SEAL-type landings the proliferation of this technology would tend to defeat those missions.