Introduction
Llana of Gathol is a book composed of four linked novellas. Michael Resnick's story, The Forgotten Sea of Mars, is a fifth novella that ties up some loose ends from Llana of Gathol: namely, where happened to Tan Hadron, and what is Hin Abtol up to?

The adventure takes John Carter to Ayathor, a hidden sea in the north pole region. There he fights a fearsome targath, is reunited with Tan Hadron, endures torture in the Chamber of Madness, makes a friend, rescues a maiden, and leads a rebellion against Hin Abtol.

Michael Resnick strikes a balance between using existing characters and creatures (John Carter, Tan Hadron, Hin Abtol, Rab-Zov, banths, and ulsios) and introducing his own inventions (Bal Daxus, Talon Gar, Lirai, targaths, Ayathor). He also name-drops a lot of characters and places, which was probably more impressive in 1965 than it is in 2020 when information is just a web search away. It's an excellent sequel to Llana of Gathol, and is one of the best pieces of Barsoom fiction to ever spring from the pen of someone other than Edgar Rice Burroughs.

On a personal note, this novella is meaningful to me because it was Mike Resnick who re-introduced me to Barsoom. I'd read the odd ERB work as a kid, but had largely forgotten him until I stumbled upon Resnick's story A Princess of Earth in the December 2004 issue of Asimov's. The story reminded me of those Barsoom books I'd heard about, so I found and read A Princess of Mars--and thus it began. I'm hooked. I've read all eleven Barsoom books (and dozens of other Burroughs books besides) and have been hunting for Barsoom pastiches and fan fiction ever since. The Forgotten Sea of Mars ranks with the best of them.

The Forgotten Sea of Mars was authorized by Edgar Rice Burroughs, Inc. and first published in 1965 as a supplement to ERB-dom #12. It's also been collected in The Worlds of Edgar Rice Burroughs, by Robert T. Garcia and Mike Resnick.

Den Valdron has written a great article about its history over at ERBzine.com. Also, ERBmania has a short note from Mike Resnick himself: Why I Wrote Forgotten Sea of Mars.

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Characters
Narrator (Michael Resnick)
John Carter
Tan Hadron, friend of John Carter
Hin Abtol, a Panar, Jeddak of Pankor and Ayathor
Rab-Zov, soldier of Hin Abtol
Panars, red men of the north pole region
Bal Daxus, dwar in Hin Abtol's army
Talon Gar, dwar in Hin Abtol's army
Dotar Sojat, alias of John Carter
Lirai, betrothed to Bal Daxus
Chosen People, gnarled dwarf-like men of the Island of the Dead
I-Pak, Jed of the Island of the Dead

Characters mentioned only:
Fo-nar, Carthoris, Kantos Kan, Tars Tarkas, Thuvan Dihn (Jeddak of Ptarth), Salensus Oil, Talu (rebel prince of Marentina), yellow men, Tavia, Dejah Thoris, Ul-to (best swordsman of Hin Abtol), Hor Vastus

Places
Zodanga
Ayathor, buried sea of the north pole region
Ayathor, city in the Sea of Ayathor
The Pits of Ayathor, catacombs and dungeons below the city
Island of the Dead, an island in the Sea of Ayathor
Tower of the Apts, part of the city of Ayathor
Chamber of Madness
Throne Room of Ayathor

Places mentioned only:
Helium, Gathol, Thuria (moon), Omean (buried sea of the south pole region), Pankor (city of the Panars), Cluros (moon), Okar

Things
radium pistols
flier
ulsio, Barsoomian rat
targath, six-limbed amphibious denizen of Ayathor, akin to a small Apt
banth, Barsoomian lion
sorapus wood
ersite, a Barsoomian stone
skeel, a Barsoomian hardwood
Zar, god worshiped by men of the Island of the Dead

Things mentioned only:
Dusar (ship), Eighth Barsoomian Ray, the Games (gladiatorial contests of Ayathor), calot

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Prologue
At a remote cabin in Arizona, John Carter appears to the narrator and tells of his latest adventures.

Chapter I: Rab-Zov
John Carter searches for his friend Tan Hadron, who he last saw during the war against Hin Abtol. An anonymous letter promises to reveal his whereabouts if John Carter will come alone to a certain boarding house in Zodanga.

Against all advice, John Carter goes. It's a trap. Rab-Zov, who wants revenge because John Carter once disgraced him in front of Hin Abtol, surrounds him with many warriors. Rab-Zov toys with John Carter by revealing that Tan Hadron is a prisoner far beneath the surface of Barsoom in Ayathor, a buried sea in the North Pole region.

John Carter escapes out a window.

Chapter II: A Unique Discovery
John Carter disguises himself by smearing his skin with red pigment, then buys a flier and heads to the North Pole region. He spots the opening to the buried sea and descends into the shaft.

Chapter III: Ayathor
John Carter arrives in Ayathor, a vast underground sea dotted with hundreds of small islands. There is a city on the largest island.

He hides his flier on a deserted island, then swims across toward the big island. A sea monster attacks him; he hacks it to death with his sword.

On the big island he's spotted by two soldiers of Hin Abtol: Bal Daxus and Talon Gar. Talon Gar draws his sword to kill John Carter, but Bal Daxus stops him. They take John Carter inside the palace, where he's recognized as the Warlord: his red paint has come off during the swim! Fifty warriors surround him.

Chapter IV: To the Pits
John Carter surrenders. He exchanges taunts with Talon Gar, who hits him in the face. John Carter challenges Talon Gar to a duel, but Talon Gar is a coward and refuses. Bal Daxus takes John Carter through the catacombs to the Pits. The catacombs are infested with ulsios. John Carter is chained in a cell next to Tan Hadron.

Tan Hadron reports that Hin Abtol is still alive and intends to conquer Barsoom: he has an army of a half million men frozen in ice, ready to be revived.

Tan Hadron describes the targath, a beast native to Ayathor: eight feet tall, six-limbed, hairy, muscular, and sightless but with an uncanny sense of perception. Also, amphibious. John Carter surmises this was the creature he encountered during his swim.

Chapter V: The Pits of Ayathor
John Carter and Tan Hadron become friendly with their jailor, Bal Daxus, who explains that Tan Hadron will be sent to the arena to compete in the Games. John Carter is destined for a worse fate.

Bal Daxus hates his jeddak Hin Abtol, but serves out of loyalty to his nation. However, things change when Hin Abtol demands to take Bal Daxus's betrothed, Lirai, as his wife. She refuses, so he imprisons her in the Tower of the Apts. Bal Daxus asks John Carter and Tan Hadron for their help to rescue Lirai, and they agree. As Bal Daxus unchains them, Talon Gar arrives.

Chapter VI: Hin Abtol
Talon Gar is suspicious of Bal Daxus but accepts his explanation that he was checking the restraints. Talon Gar takes John Carter to see Hin Abtol, who sentences him to death in the Chamber of Madness.

Chapter VII: The Chamber of Madness
The Chamber of Madness is a circular room with twelve securely bolted doors evenly spaced along the wall. There is no ceiling, only a gaping darkness broken by a few beams of sorapus wood. John Carter is given his longsword, but there is nothing to use it against.

When he attempts sleep, a blinding spotlight and a loud metallic grating sound wake him. The room's temperature oscillates between unbearable heat and freezing cold. His thirst grows intense, but he's given only vinegar to drink.

His torturers release chained banths into the Chamber, one from each door, and the only space beyond their reach is the precise center of the chamber. John Carter draws his sword and blinds the banths by putting out their eyes.

Chapter VIII: The Secret Passage
The banths are pulled back. A loud voice mocks John Carter. He undergoes the cycle of torture--light and sound, heat and cold--for days. He pretends to go mad.

His captors release a targath into the Chamber of Madness. Weakened as he is, John Carter kills it with his longsword. Then he jumps--a twenty-foot vertical leap into the rafters.

He climbs up through the rafters and soon locates a secret passageway behind a panel. He follows the corridor for miles before taking refuge in a long-deserted store-room. He kills and eats an ulsio, then sleeps. Once refreshed, he returns to the corridor where he runs into Bal Daxus. Bal Daxus explains that this secret passageway leads to the Tower of the Apts, where Lirai is imprisoned. John Carter agrees to help Bal Daxus rescue Lirai from the tower, then they will both go release Tan Hadron from the Pits.

Chapter IX: Reunion and Pursuit
At the Tower of the Apts, Bal Daxus distracts the guards, then he and John Carter kill them. They rescue Lirai, then leave her in the deserted store-room while they go to release Tan Hadron.

At the Pits, Bal Daxus dismisses the guards, pretending that he's come to relieve them. They release Tan Hadron, then return to fetch Lirai--but she's missing. There are signs of a struggle and footprints leading down a corridor. They follow the corridor all the way to the shore of the Sea of Ayathor. They see Talon Gar has abducted Lirai and taken her aboard a raft: they are headed for the Island of the Dead. The heroes build a raft and follow.

Chapter X: The Island of the Dead
At the Island of the Dead, John Carter and friends meet a race of gnarled dwarf-like men. These people have no knowledge of the outside world beyond Ayathor. They worship a god named Zar. Every native of the Island of the Dead is old.

Their leader, I-Pak, shows off his people's treasure: sculptures of yellow and red warriors, posed as if in battle. The sculptures are made from living men preserved in a waxy substance. I-Pak and his fellows have already captured Talon Gar and Lirai. I-Pak decrees that Lirai will be added to the sculpture garden. Of the men, they will fight duels until only one remains, then he will be preserved in wax.

In the first duel, John Carter kills Talon Gar. The second duel is between John Carter and Tan Hadron, but they join forces and attack their captors--who are foolishly unarmed. I-Pak surrenders. The four heroes leave the Island of the Dead and go to retrieve John Carter's flier--but it's been smashed! They are trapped in Ayathor.

Chapter XI: The Frozen Army
They return to the city. Bal Daxus leads John Carter to the chamber where Hin Abtol keeps his frozen army. Bal Daxus revives them, and John Carter addresses the army. The men have no love for Hin Abtol and agree to help John Carter defeat him.

John Carter leads the army into the Throne Room. He fights his way toward Hin Abtol, who escapes out a secret passage behind the throne. Pausing only to kill Rab-Zov, John Carter pursues Hin Abtol into the catacombs.

Chapter XII: Adventure's End
Hin Abtol's forces are defeated. John Carter catches Hin Abtol and kills him. Bal Daxus is acclaimed Jeddak of Pankor and Ayathor. He marries Lirai. John Carter and Tan Hadron return to Helium.

Epilogue
John Carter hints at further adventures, but must return to the woman he loves. The End.