Carter runs and stumbles through the inky tunnels.  He didn’t make an effort to leave the High-Priest’s chamber by the same route they came in, because the frescoes he saw on the way in were just too scary.  However they were like a child’s garden, compared to the frescoes he sees running randomly!  Lovecraft can’t even describe these frescoes, that’s how bad they are.  Those he now saw were even more horrible than those he had seen then, and he knew he was not in the corridors leading outside.

Eventually his lamp runs out of fuel, and he’s left with feeling his way forward on his hands and knees.  He prays to the Great Ones for aid, but no miraculous night-gaunts rescue him (or whatever Carter was angling for there).  There’s a whole Bilbo-lost-in-the-goblin-tunnels sequence, of Carter aimlessly crawling along in the dark, sometimes uphill, sometimes downhill, once over a step that doesn’t seem need to be there.  Always it’s a little more humid than before, but Carter doesn’t know what to make of that.  He figures he’s ended up tunnels below the stony plain of Leng itself, and just as he might be starting to get his bearing, he falls down a bottomless shaft.

Okay, not really bottomless.  Also not really a shaft, it’s apparently simply a very steep tunnel.  Of the length of that hideous sliding (little-known fact: That Hideous Sliding was CS Lewis’s first-draft title for the conclusion to his SF trilogy) he could never be sure, but it seemed to take hours of delirious nausea and ecstatic frenzy. Then he realized he was still, with the phosphorescent clouds of a northern night shining sickly above him.

Carter has somehow managed to slide all the way down from Leng to the land below the plateau.  You can tell he’s no longer in Leng because the pavement on which he lay was pierced by straggling grass and wrenched asunder by frequent shrubs and roots.  All around him are ruins: broken columns, wreckage that was once buildings, smashed flagstones, all that stuff.  Behind him a basalt cliff carved with vile symbols stretches up and up.  Before him, the columns outline a great plaza.  In the plaza, and over it, are two huge statues of lions, carved from diorite.  Carter recognizes the lion statues from the hideous frescoes he saw in the previous entry, and knows that he stands in the ruins of ancient Sarkomand.

He plugs up the tunnel he just slid out of, using debris, and then takes stock.  His thoughts run like so.

1) He does not know how to get from Sarkomand to Kadath, or for that matter from Sarkomand to anywhere.

2) The ghouls who helped him (back in Part 13 or so) planned on returning to the Underdark via Sarkomand, but Carter doesn’t know whether they made it to here, or how.

3) In theory he can get from Sarkomand to the Underworld to the land of the ghouls to the city of the Gugs to the Enchanted Wood to the Oukianos valley to the Cerenian Sea to Celephais to Inquanok to, eventually, Kadath.  That’s a lot of retracing of his steps, though.

4) The hideous frescoes (vide supra) indicated that Sarkomand was not far from the barren island where the moon/frog/shapeshifters dwelled.

5) If he had a boat he could maybe get from Sarkomand to that island to Inquanok.

6) He does not have a boat.

7) Nor is he likely to just find one randomly.

8) Crap.

As he grumbles and recaps to himself, Carter spots a distant firelight.  He isn’t alone in Sarkomand!  The glow rose and fell fitfully, flickering with a greenish tinge which did not reassure the watcher, Lovecraft tells us, in case we had assumed Carter was about to meet new friends.  He creeps towards the light, passing through deserted streets and galleries, and sees a big bonfire at the edge of the old wharves, near a moored black ship (either the same black ship that took him to the Moon, or one so like it that it doesn’t matter).  There’s a smell, and some shapes huddled near the fire.

Carter wants nothing to do with any of this.  He starts to creep away from the scene, thinking to himself about how when he decided he was unlikely to find a boat, he hadn’t considered a Moon boat.  Moon boat may as well not be a boat at all, really.

But then he hears the frightened meeping of a ghoul!  Soon it’s a veritable chorus of anguish! Carter isn’t made of stone.  He turns back, and this is a good bit for quoting Lovecraft directly:

Once in crossing an open street he wriggled worm-like on his stomach, and in another place he had to rise to his feet to avoid making a noise among heaps of fallen marble. But always he succeeded in avoiding discovery, so that in a short time he had found a spot behind a titan pillar where he could watch the whole green-litten scene of action. There around a hideous fire fed by the obnoxious stems of lunar fungi, there squatted a stinking circle of the toadlike moonbeasts and their almost-human slaves. Some of these slaves were heating curious iron spears in the leaping flames, and at intervals applying their white-hot points to three tightly trussed prisoners that lay writhing before the leaders of the party. From the motions of their tentacles Carter could see that the blunt-snouted moonbeasts were enjoying the spectacle hugely, and vast was his horror when he suddenly recognised the frantic meeping and knew that the tortured ghouls were none other than the faithful trio which had guided him safely from the abyss, and had thereafter set out from the enchanted wood to find Sarkomand and the gate to their native deeps.