Shannach- The Last — Leigh Brackett

An Earthman on Mercury stumbles into a long lost colony in a hidden air-filled valley, ruled by harsh Sunstone wielding hawk controlling lords - and, of course, an alien overlord behind them.

excerpt

It was dark in the caves under Mercury. It was hot, and there was no sound in them but the slow plodding of Trevor's heavy boots.

Trevor had been wandering for a long time, lost in this labyrinth where no human being had ever gone before. And Trevor was an angry man. Through no fault or will of his own he was about to die, and he was not ready to die. Moreover, it seemed a wicked thing to come to his final moment here in the stifling dark, buried under alien mountains high as Everest.
He wished now that he had stayed in the valley. Hunger and thirst would have done for him just the same, but at least he would have died in the open like a man, and not like a rat trapped in a drain.

Yet there was not really much to choose between them as a decent place to die. A barren little hell-hole the valley had been, even before the quake, with nothing to draw a man there except the hope of finding sun-stones, one or two of which could transform a prospector into a plutocrat.

Trevor had found no sun-stones. The quake had brought down a whole mountain wall on his ship, leaving him with a pocket torch, a handful of food tablets, a canteen of water, and the scant clothing he stood in.

He had looked at the naked rocks, and the little river frothing green with chemical poisons, and he had gone away into the tunnels, the ancient blowholes of a cooling planet, gambling that he might find a way out of the valleys.

Mercury's Twilight Belt is cut into thousands of cliff-locked pockets, as a honeycomb is cut into cells. There is no way over the mountains, for the atmosphere is shallow, and the jagged peaks stand up into airless space. Trevor knew that only one more such pocket lay between him and the open plains. If he could get to and through that last pocket, he had thought . . .
But he knew now that he was not going to make it.

He was stripped to the skin already, in the terrible heat. When the weight of his miner's boots became too much to drag, he shed them, padding on over the rough rock with bare feet. He had nothing left now but the torch. When the light went, his last hope went with it.
After a while it went.