Like many (most?) things that make the rounds on the Internet ("Find what you love," anyone?), the Shackleton ad is not real.
In advertising anthologies, one prize-winning entry crops up repeatedly. It was said to have appeared in The Times of London in early 1907:
MEN WANTED FOR HAZARDOUS JOURNEY. LOW WAGES, BITTER COLD, LONG HOURS OF COMPLETE DARKNESS. SAFE RETURN DOUBTFUL, HONOUR AND RECOGNITION IN THE EVENT OF SUCCESS.
The copy has been singled out as a masterpiece of deadly frankness.
Unfortunately, it seems that the advertisement is as spurious as it is famous: It has never appeared in any newspaper anywhere. Regardless, the conditions it implies, as well as the spirit it conveys, are so hauntingly evocative that I extracted from it the title for this book.
-John Maxtone-Graham, Safe Return Doubtful, p. xii