Image copyright F.HURLEY/SPRI Image caption The Endurance's ill-fated voyage marked the end of the "heroic age" of Antarctic exploration

The attempt this week to find Sir Ernest Shackleton's missing ship, the Endurance, has ended - without success.

A UK-led expedition to the Weddell Sea sent a sub to the ocean floor to look for the sunken polar yacht, but this robot was itself lost in the process.

The team has now withdrawn from the area because of deteriorating weather and sea-ice conditions.

Shackleton and his crew were forced to give up the Endurance in 1915 when frozen floes crushed its hull.

Their escape across the Antarctic sea-ice on foot and in lifeboats is an astonishing story of fortitude and survival.

The idea of finding the remains of the Endurance has captivated maritime historians and archaeologists for decades.

"As a team we are clearly disappointed not to have been successful in our mission to find Endurance," said Mensun Bound, the director of exploration for the Weddell Sea Expedition 2019 group.

"Like Shackleton before us, who described the graveyard of Endurance as 'the worst portion of the worst sea in the world', our well laid plans were overcome by the rapidly moving ice, and what Shackleton called 'the evil conditions of The Weddell Sea'."

What happened during the search this week?

The team, on its South African ice-breaker, the SA Agulhas II, arrived at the last-reported position of Endurance on Sunday.

The researchers immediately set about putting down an Autonomous Underwater Vehicle (AUV) to go map a wide grid in waters that are 3,000m deep. But just as this submersible was coming to the end of its more than 30-hour dive, the communication link to the Agulhas failed.

It is not clear if this was the result of just the difficult sea-ice conditions, or component failure on the sub, or even the robot colliding with an obstruction. Although the last possibility seems very unlikely given how flat the seafloor is known to be in this region of the Antarctic.

It is conceivable of course that by the time of the break in communications, the sub had already captured images of Endurance in the Weddell sediments - but that will never be known. An AUV has to be recovered first for its scan data to be pulled off and examined.

The US company Ocean Infinity ran the AUV dive. Its representative Oliver Plunkett said: "Everyone at Ocean Infinity is deeply disappointed that at the eleventh hour, we were not able to produce the images of what is without doubt the most challenging shipwreck in the world to locate.

"We understood the risks of pushing the boundaries of what's been done before with technology operating in the harshest environment on the planet. Our team worked tirelessly throughout and are rightly entitled to celebrate what they achieved in advancing knowledge and understanding."

Why is the team not too downhearted?

The search for Shackleton's ship was an "extra" for the Weddell Sea Expedition 2019. Its main purpose for being in the region was to study the nearby Larsen C Ice Shelf, which in 2017 calved the monster iceberg known as A68.

Understanding the climate interactions in this part of the world is imperative, and the Larsen investigation, completed at the end of January, is said to have been hugely productive, with an AUV acquiring remarkable imagery of the seafloor under the shelf.

Expedition chief scientist Prof Julian Dowdeswell commented: "Through the scientific data gathered during the expedition, we have deepened our knowledge and understanding of Antarctic oceanography and ecosystems, and our observations on the glaciology and geology will play a critical role in our understanding of Antarctic ice shelves and sea-ice and, importantly, the changes that are occurring here today."

Image copyright WEDDELL SEA EXPEDITION 2019 Image caption The icebreaker SA Agulhas II has been the base for the survey

The story of Shackleton's ill-fated 1914 Imperial Trans-Antarctic Expedition has become the stuff of legend, and is even used at business schools in examples of different approaches to staff management.

The Irish-born explorer had wanted to make the first land crossing of the Antarctic continent. He knew it would be tough, which prompted the now famous crew recruitment advert:

"Men wanted for hazardous journey. Low wages, bitter cold, long hours of complete darkness. Safe return doubtful. Honour and recognition in event of success." (Although often cited, there are doubts that this advert ever appeared.)

The expedition never got to begin its traverse because the Endurance was captured by the Weddell's thick floes in early 1915.

The 44m-long steam yacht drifted for nine months before the pressure of the ice holed the hull and floodwater took it under.

Shackleton and his 27-man crew made their escape northwards, dragging their lifeboats across the pack ice in those places where they couldn't sail on the sea surface.

They managed first to get to Elephant Island, at the tip of the Antarctic Peninsula, from where Shackleton then set off with five others to try to reach South Georgia to get help.

He succeeded, despite having to navigate across 1,000km of the Southern Ocean in a tiny boat.

Even when Shackleton arrived at the British Overseas Territory, he had to climb a set of mountains because the whaling stations that would come to his aid were on the far side of the island.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos