A sharp-eyed user of Microsoft's Virtual Earth has found images of classified US Navy technology.

The image shows an Ohio-class ballistic missile submarine being pulled out of the water at the Trident Submarine Base in Bangor, Washington and displays its propeller in full view.

Submarine propellers are highly classified and are usually hidden when submarines go into dry dock.

Dan Twohig, a deck officer on the Washington state ferry service, found the image when he was searching for a new house. He also writes a blog about shipping called Monster Maritime, on which he broke the news of the discovery.

"The prop photos were not taken from a satellite but by an airplane on a mapping run of the area," he wrote.

"I am certain that I am not the first person to see this photo, but I was just the first person to point it out and say: 'Whoa, I don't think that the military is going to be happy with this.'

"Satellites are one thing, but frankly, I am surprised that the air space over sensitive bases like Bangor is not better controlled."

Details of submarine propellers are highly secret, since they are the primary noise producing part of the submarine during cruising. Modern designs are built to reduce cavitation, the process by which bubbles form as the blades turn underwater.

A similar case occurred early this year when Google Earth picked up the first public pictures of a Chinese ballistic missile submarine.