The US Navy is set to unleash an army of 'ghost drones' to scour the coasts for enemy submarines.

They hope to end the growing threat of quiet, diesel powered enemy submarines enters American waters undetected.

The robot boats will go to sea for us to three months at a time.

The Anti-submarine Warfare Continuous Trail Unmanned Vessel, or ACTUV will be able to operate for several months at a time scouring the seas and coastal areas for silent, diesel powered enemy submarines.

THE SILENT ENEMY SUBS Diesel-electric submarines, with their nearly-noiseless engines, are incredibly difficult to track from afar. They're also cheap at $200 million to $300 million apiece, making them affordable to the likes of Iran, which claims to have a fleet of 17. Advertisement

The project began in 2010, when the Defense Advanced Research Projects Agency, or DARPA, announced that they were building a 132-foot autonomous boat to track quiet, diesel-powered submarines.

The program was dubbed Anti-submarine Warfare Continuous Trail Unmanned Vessel, or ACTUV.

In six weeks of tests along a 35-nautical mile stretch of water off of Mississippi earlier this year, testers at engineering company Leidos and DARPA put the ACTUV's systems through 100 different scenarios.

The test boat was able to tail a target boat at 1 kilometer's distance, something military bosses say is a major step forward.

'Picking up the quiet hum of a battery-powered, diesel-electric submarine in busy coastal waters is 'like trying to identify the sound of a single car engine in the din of a major city,' says Rear Admiral Frank Drennan, commander of the Naval Mine and Anti-Submarine Warfare Command.

Speaking at a National Defense Association Event in Virginia,DARPA program manager Ellison Urban outlined why the Navy needs sub-hunting boat bots.

The test boat was able to tail a target boat at 1 kilometer's distance, something military bosses say is a major step forward.

'Instead of chasing down these submarines and trying to keep track of them with expensive nuclear powered-submarines, which is the way we do it now, we want to try and build this at significantly reduced cost.

'It will be able to transit by itself across thousands of kilometers of ocean and it can deploy for months at a time.

'It can go out, find a diesel-electric submarine and just ping on it,' said Urban.

Diesel-electric submarines have nearly-noiseless engines, are incredibly difficult to track from afar.

In six weeks of tests along a 35-nautical mile stretch of water off of Mississippi earlier this year, testers at engineering company Leidos and DARPA put the ACTUV's systems through 100 different scenarios.

Price tags ranging from $200-$300 million put diesel-electric subs within reach of smaller, volatile countries.

Russia has been selling diesel-electric subs to buoy its shipyards, triggering what some are calling an undersea arms race.

Reportedly, Algeria has ordered two, Venezuela is expecting five, and Indonesia will have six subs by 2020. Iran claims to have a fleet of 17 diesel-electric subs.

To spot the threat, Leidos developed an unarmed, unmanned vessel to shadow diesel-electric subs for months across thousands of miles of ocean and chase them out of strategic waters.

'Called the ACTUV, the unmanned boat can be deployed for months and track underwater threats for thousands of miles without human contact.

'It keeps our troops out of harm's way and also minimizes risks to the marine ecosystem by limiting the use of sonar,' the firm says.

It claims diesel-electric submarines are quickly becoming one of the biggest threats to naval operations and a $1.8 trillion commercial shipping industry.