SHACKLETON. By Roland Huntford. Atheneum. 774 pp. $29.95.

ENOUGH LIFE and money has been spent on this sterile quest. The Pole has already been discovered. What is the use of another expedition?" So wrote Winston Churchill, First Lord of the Admiralty, in response to Sir Ernest Shackleton's plea for government help with his proposed crossing of the Antarctic continent. On another occasion the First Lord was even more peppery, referring to Shackleton and his crew as "these penguins."

Churchill has a point. The South Pole had been reached not once but twice, two years earlier -- first in late 1911 by the methodical Norwegian Roald Amundsen and then a few weeks later by the feckless Englishman Robert Falcon Scott. Shackleton's expedition had little to recommend it beyond the dubious goal of admitting its leader into the inner ring of polar explorers. On the other hand, no expedition led by Shackleton could fairly be described as "sterile." Words like "risky," "thrilling" and even "preposterous" would be more like it. Aside from Stanley in Africa, perhaps no explorer has ever equalled Shackleton in risks taken, hardships weathered and ultimate greatness so narrowly missed. It was almost as if he orchestrated his career, so that each of his polar expeditions attained a higher pitch of danger than the one before it. Nor have many explorers been the subject of so rich and riveting a biography as this one by Roland Huntford. Never one to lack for a grand phrase, Shackleton found one perfectly suited to confute naysayers like Churchill: "It will be the last great expedition," he predicted, "that can hope to discover large tracts of land."

The man who penned that imperialistic appeal was born to a middle-class Anglo-Irish family in 1874. He went to Dulwich College, whose other distinguished alumni include P.G. Wodehouse. Shackleton and his brother Frank both seethed with ambition, but in Frank's case the energy went awry. A professional scapegrace who specialized in promoting worthless stock issues, in all likelihood he also stole the Irish crown jewels. Ernest put the same kind of charm to more legitimate use, in fund-raising for his adventures.

He began his preparations for exploring as an indentured seaman. In 1900 he volunteered for the National Antarctic Expedition, during which he took the measure of Scott and reckoned himself the better man. The two were united, however, in their disdain for planning and their adherence to what author Huntford calls "the national myth that improvisation was best."

In practice, this credo meant that British explorers declined to bring along dogs to haul their sledges or skis to ease their way. On separate expeditions Shackleton and Scott each committed the egregious folly of using ponies as beasts of burden. (All of the poor creatures perished.) Shackleton even lugged an automobile along on one trek; it started but promptly bogged down in snow. He at least learned from his errors: for his third expedition he imported plenty of dogs and imposed a fresh-food diet to ward off scurvy. Scott's stubborn reliance on manhauling and his ignorance of scurvy ensured his death on the way back from the South Pole in 1912.

Another typically British attitude, the exaggerated sense of fair play, hamstrung Shackleton on his second polar expedition, in 1907. Scott had already announced plans to lead a poleward trek whose base would be situated at one of the continent's safest harbors, McMurdo Sound. Under moral and financial pressure, Shackleton signed an agreement promising to steer away from the sound and to land elsewhere. In the event, climate conditions made the promise impossible to keep: Shackleton ultimately had to find haven in McMurdo Sound. Scott made ugly noises about the "betrayal" for the rest of his short life -- but quietly incorporated Shackleton's daring route over the Beardmore Glacier into his own polar journey a few years later.

SHACKLETON almost reached the South Pole that time. Only the sheer mathematics of food supply prompted him to order a retreat 100 miles short of the shimmering goal. "It was arguably one of the bravest acts in the history of exploration," Huntford writes. "Any fool can go blindly forwards. It required insight and moral courage to turn back; especially when it meant publicly admitting defeat and to a despised rival."

Shackleton managed to put together his transpolar expedition -- the one that so irked Churchill -- only by dint of his great panache. He was obliged to do a lot of fast talking in the drawing rooms of wealthy women. As he would pace up and down, pouring out plans, "The eye, as usual," Huntford notes, "was watching what the heart was saying." Once again he left his wife, mistress, childr, and the financial predicaments that he could never resolve for a journey to the end of the earth.

This expedition was a failure on the colossal scale where only Shackleton was nervy enough to work. The plan was for ship one to land men at McMurdo Sound, from where they would hike inland, depositing caches of food for Shackleton and his cohorts. The latter were on ship two, Endurance, which was to land them at the opposite edge of the continent.

The first part of the plan came off fairly well until ship one was blown out to sea and prevented from reentering the sound by the onset of the Antarctic ice pack, which left four men marooned ashore in a hut. On the other side of the continent, ice was even more treacherous. It closed in on Endurance, held her fast, and gradually demolished her like a squeezed egg. Shackleton and his men escaped in the ship's boats and huddled for weeks on drifting ice. Fifteen hundred miles of drifting later, when weather finally permitted, they managed to sail the boats to Elephant Island. There two boatsful of men remained while in mid-winter Shackleton and a crew of six set sail for the manned whaling stations at South Georgia Island, a distance of 700 miles over the world's roughest seas.

They almost foundered innumerable times. "Nearly always there were gales," Shackleton reminisced. "So small was our boat and so great were the seas that often our sail flapped idly in the calm between the crests of two waves. Then we would climb the next slope and catch the full fury of the gale where the wool-like whiteness of the breaking water surged around us."

Incredibly, they made it. Once again Shackleton subdivided his party, leaving four exhausted men on one side of the island while he and two strong companions hiked 20 miles over snow-covered mountains to fetch help. It was on this trek that he felt a benevolent fourth presence alongside them -- an image that T.S. Eliot borrowed for a striking passage in The Waste Land. After Shackleton and company reached the whaling station -- their first glimpse of civilization in a year-and-a-half -- it took them months to gather the expedition's far-flung human pieces. Not a man under Shackleton's direct command was lost in all this multiplicity of perils -- a record on which he prided himself. The ordeal ruined Shackleton's always spotty health, and he died in 1922 of a heart attack, at the outset of still another Antarctic quest.

Roland Huntford has also written Scott and Amundsen, a dual biography of the South Pole conquerors whose paths Shackleton crossed (figuratively and literally) so many times -- and the basis for the recent Masterpiece Theatre series, The Last Place on Earth. That was a long book, but this one is even longer. Yet Huntford wastes no ink: by page 50 he has already gotten Shackleton inside the Antarctic Circle for the first time. Clearly and colorfully written, intelligently organized, shrewd in its judgements of men and its assessments of risks, vibrant with narrative brio, Shackleton is a splendid performance.