The "world's most advanced torpedo" is almost ready for service with the Royal Navy after extensive trials in Scotland.
The Spearfish - which has been British submarines' weapon of choice since the early 1990s - is undergoing a £270m upgrade which includes a new warhead, safer fuel system and an enhanced electronic "brain".
Around 100 engineers and experts from BAE Systems in Portsmouth have spent nearly six years improving the torpedo, the Navy said.
During its penultimate trial, it was "fired" repeatedly at frigate HMS Sutherland as scientists, engineers and sailors studied its performance.
The trial took place over four days on special ranges near Kyle of Lochalsh which is 10km long, 6km wide, up to 200 metres deep and peppered with state-of-the-art sensors.
The torpedo's software and hardware enhancements were tested, while Plymouth-based HMS Sutherland did its best to fend off the weapon's attacks, using a mix of evasive manoeuvres and advanced acoustic counter-measures to lure it away.
Spearfish was set to "run deep" for safety reasons, so the "battle" was played out on the displays in HMS Sutherland's operations room, where the shrill sound of whistles announced a torpedo in the water.
The Navy said the warhead is six times more powerful than that carried by the smaller Sting Ray torpedo, fired by ships such as Sutherland or launched from Merlin and Wildcat helicopters.
It can break the back of frigates, destroyers, and similar-sized warships, and take out any underwater threats, the service added.
"During the trial this week we have put our elite training into action, using a variety of underwater sensors to locate and track the weapon," said Able Seaman Matthew Brown, one of the underwater warfare specialists who has been tracking Spearfish.
"Having one of the most advanced and capable torpedoes in the world fired at you certainly puts the pressure on."
It will be introduced to frontline hunter-killer and nuclear-deterrent submarines over the next three years and will be in service into the 2050s.
A final trial of Spearfish will take place at British Underwater Test and Evaluation Centre later in 2020 before the weapon is declared operational and delivered to the submarine fleet.
Cover image: The upgraded Spearfish (Picture: Royal Navy).