Cloaked submarines

COSMOS magazine reported They're already masked beneath the waves, but soon these silent killers could be harder to detect - even using sonar. American scientists have proven that a cloaking device which would render submarines and ships invisible to sonar is theoretically possible. "We've devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves," said Steven Cummer, an electrical and computer engineer at Duke University in Durham, North Carolina.

I understand the Romulans and Klingons already have this technology.