A Yank at Valhalla by Edmond Hamilton (1973)
After reading The Haunted Stars, a 1960 space opera presenting the "mature" side of Edmond Hamilton, I was in the mood for an older, more pulpish Hamilton and this book filled the bill. First published in Startling Stories in January 1941, this short novel lingered in pulp purgatory in this country until it was published as one half of an Ace Double in 1973 and it's a slam-bang pseudo-science thiller with non-stop action. (It was published in England as a 1950 paperback under the title of The Monsters of Juntonhein and in Germany in 1958 as Unternehmen Walhalla.)
Keith Masters, a physicist accompanying an arctic expedition, is on a trawler making its way through the Actic Ocean when he gets an unexplainable feeling and directs the crew to lower a net into the sea. The net comes up with a golden rod, incredibly old and marked with runes similar to Old Norse. He decides to carry the rod with him on an reconaissance flight in his small rocket plane. Heading north, he finds himself in the eye of a heavy -- and seemingly impossible -- gale, driving him foward for an equally impossible six hours. Also impossible is the land mass below him where he is forced down, a land mass with castles and a fantastic rainbow bridge - the land of the Aesir!
So heere comes the pseudoscience, the old Norse gods exists, but they are not gods. They are what is left of the first race of men who lived in underground cavern where they developed an intricate society and an advanced science. One scientist, Loki, tried to achieve immortality and in doing so released deadly radiation throughout the underground world. Now immortal (or at least pretty well long-lived) the Aesir advanced to the surface led by their chieftain Odin. Small in number, they established their city in the arctic, using their advanced science to hide themselves from the outer world. Loki, in an attempt to overthrow Odin, sided with the Jotan, one of the two barbarous races that occupied Midgard, the land where the Aesir settled. (The other race was the Alfings, a race of cave dwellers and artisans.) Loki and the Jotans were ultimately defeated and Loki as well as his genetically modified pets Fenris, the giant wolf, and Iormungandr, the poisonous Midgard serpent, were place in suspended animation and locked behind an energy shield. The shield can only be opened by a key, You guessed it...the key was the golden rod that Masters had fished out of the ocean depths. The Aesir, horrified by what Loki's science had done, denounced science.
Loki may be in suspended animation, but his mind is still alert and had been using its pyschic powers to influence people. The Jotans are waiting for him to arise and led them in victory over the Aesir, and with Loki's scientific knowledge, this final vistory is all but assured.
There's fights and battles and derring-do. And there's the beautiful, passionate niece of Odin who falls instantly in love with Masters, and vice versa. (Not to worry. Instead of being millenia old like the other Aesir, she's only twenty. Conenient and way more palatable than a May/mega December romance.) And there's monsters, swordplay, Alfings, and sheer pulp fun.
The Ace Double, by the way, was backed with Ross Rocklynne's The Sun Destroyer, a frist edition, fix-up novel of the four stories that comprised his Darkness series (1940-1951). This one looks to be another good read and I'll be getting to it pretty soon.