The Fabulous Riverboat by Phillip Jose Farmer

They were out of the cave, in the open air, on the side of the mountain. Several thousand feet below them was a sea of clouds. The sun was hidden behind the mountains, but the sky above was not yet dark. The narrow ledge continued, and they crawled on their bloodied hands and knees downward now, since the ledge had narrowed to nothing.

Trembling, they clung to the tiniest of fingerholes. A man slipped and fell and clutched another man! Screaming, both disappeared into the clouds.

The air became warmer.

“The River was giving up its heat,” Clemens said. “It not only originates at the north pole, it also empties there after picking up heat in its serpentine wanderings over the entire planet. The air at the north pole is cold but not nearly as cold as that on Earth. This is all speculation, of course.”

The party came to another shelf on which they could stand, facing the mountain, and proceed sidewise, Like crabs. The shelf curved around the mountainside. Joe halted. The narrow valley had widened into a great plain. He could hear, far below, the dash of surf against rock.

Through the twilight, Joe could see the mountains ringing the sea of the north pole. The cloud-covered waters formed a body about sixty miles in diameter. The clouds were thicker at the opposite end of the sea. He didn’t know why then, but Sam had explained that the clouds hid the mouth of The River, where the warm waters came into contact with the cold air. Joe took a few more steps around the curve of the ledge.

And he saw the gray metal cylinder sitting on the path before him.

For a moment, he did not understand what it was, it looked so alien. It was so unexpected. Then it flowed into familiar lines, and he knew it was a grail left by a man who had come before him on this dangerous path. Some unknown pilgrim had survived the same perils as he. Up to that point, that is. He had put the grail down to eat. The lid was open, and there was the stinking remnant of fish and moldy bread within it. The pilgrim had used the grail as a pack, perhaps hoping he might come across a grailstone and recharge it.

Something had happened to him. He would not have left the grail there unless he had been killed or had been so frightened he had run away without it. At this thought, Joe’s skin chilled.

He went around the outcropping that was sitting at a point where the ledge went around a shoulder of granite. For a moment, his view of the sea was blocked. He went around the outcropping—and he cried out.

he men called and asked what troubled him.

He could not tell them because the shock had taken away his newly learned speech, and he spoke in his native tongue.

The clouds in the middle of the sea had roiled away for a few seconds. The top of a structure projected from the clouds. It was cylindrical and gray, like the top of a monster grail.

Mists rose and fell around it, now revealing, now veiling.

Somewhere in the mountains ringing the” polar sea, a break existed. At that moment, the low sun must have passed this notch in the range. A ray of light fell through the notch and struck the top of the tower.

Joe squinted his eyes and tried to see into the brightness of the reflection.

Something round had appeared just above the top of the tower and was settling down toward it. It was egg-shaped and white, and it was from this that the sun was sparkling. The next instant, as the sun passed by the notch, the sparkle died. The tower and the object above it faded into darkness and mist. Joe, crying out at the sight of the flying object, stepped back. His leg struck the grail left by the unknown pilgrim.

He swung his arms to regain his balance, but not even his apelike agility could save him. He toppled backward, bellowing horror as he turned over and over. Once he glimpsed the faces of his companions, a row of dark brown objects with the darker O’s of mouths, watching his descent to the clouds and waters beneath.