Although I'd imagine such a sophisticated dwarven civilization would have legendary dining halls to quench tantrum spirals... It would be far more interesting if the xenoarcheologists found the dwarves safe and sound underground
Initial surveys had classified this as a barren world - sulphurous atmosphere, signs of heavy vulcanism in the planet's past, no life-signs detected on the surface apart from a few resilient algae in the steaming mineral sludge that comprised the shallow oceans.
It was a prospecting mission found them - boring through the old long-cooled lava flows in search of any minerals worth extracting, when one of their core samplers broke through into a vast cavity over a hundred metres below the surface. They tried again a little way away - except this time rather than breaking through, the sampler brought up... artifacts. Or fragments of artifacts, at least. Carved and worked stone, metal and glass.
After that they brought in the xenoarchaeologists, did some deep-level scanning, and discovered a whole network of cavities under there - not the mindless formations of erosion and subsidence, but structures - corridors, hallways, rooms and chambers laid out with orderly deliberation, carved into the bedrock itself. Investigation of other likely-looking sites brought back similar results. Imagine Pompeii or Herculaneum, but engulfing New York, London, Beijing and Rio de Janeiro all at once.
An entire civilisation, buried under hundreds of metres of solidified lava and volcanic ash.
At first we thought it might be some kind of mass-eruption, some tectonic disaster that had immolated and buried the entire planetary surface. That the populace had sought refuge in subterranean cities implied that perhaps they'd seen it coming, had hastily carved out these shelters and planned to wait out the cataclysm, so they had seismologists, vulcanologists, geologists, experts on planetary geophysics come and find out what happened - and whether it could happen again. So they started looking at faultlines and the old cindercones of the world's volcanoes, and the xenoarchaeologists sent a team into the ruins to start exploring and cataloguing.
Then a team of the xeno-whatevers disappeared. At first we chalked it up to an accidents. Ancient tunnels, abandoned for who knows how long. Makes sense that some would collapse. So we sent some drills to recover the bodies. Only the tunnel wasn't collapsed. We found the bodies, slashed and stabbed. What had been a recovery after an accident became a foray into hostile territory. The drill were withdrawn. Soldiers were deployed. They kept up a live video feed to base camp. They went in there with their assault rifles and their grenades. We saw the feed. Cyan juggernauts, with beady eyes looking out through the only holes in their helmets. Bullets bounced right off the stuff. The aliens had axes. It was all they needed. The soldiers were down in seconds. The whole expedition lasted less than a day.
Well, the suits decided they needed that blue stuff, and they'd throw away as many lives as they needed to in pursuit of the crap. So they send in the big guns. The servicemen call them bomb-bots. Just a bomb an eye, and treads. They loaded one up with as much non-nuclear as they could get approved, and sent it in there. As soon as it saw one of the blue things, they blew it right up. Now, any school child can tell you that stone doesn't incinerate easily. So the place wasn't open, and they sent in another drone. And what do they see when they get there? The creatures, taking chisels and picks to the walls of the new chamber. The one they killed - or they figured they killed - was gone. Now, when the creatures saw the drone, they went and chopped the thing in half. But they didn't disable the cam, and that's how we got to know the stuff about them we know today.
See, the bot was brought down, down into the darkness. We started seeing them without the cyan armor on. And they looked just like us! Well, not exactly, they were shorter, wider. They all had beards, and their eyes were every color you could think of. But the important bits were the same. That video got out on the internet, and suddenly everybody wanted to talk to the guys. Let them know we were intelligent. Not for killing. But how?
Well, they had a spoken language. From gestures, we got a pretty good idea of how to talk about trade. So we sent in a volunteer. The guy must have had balls of steel, but he went. We sent a truck full of anything we could think of with him. Cloth, animals, tools, some raw materials, Different types of food. He saw one of their blue-clad juggernauts. He talked to them, made the sounds we thought were about trade. The thing led him to a big empty room, and left. Soon others came with all manner of goods.
These creatures had a sophisticated culture, and their skill in crafting was unparalleled - you can see the artifacts for yourself, there's plenty of them on display in the Colonial Museums on a dozen planets. Scenes from daily life, historic events, that kind of thing.Exquisitely-worked, sublime in their artistry. Made the caches in the Valley of the Kings look like junk-shop detritus. They didn't want much of what we had though. Took some wood, some of the more unusual metals. All the animals. Nothing else. Gave us their handiworks generously. Gave us some sort of roasted mushroom.
These days? Well, they're one of our best trading partners. Give us our wonderful kitsch, we supply them with the material. We're making a killing selling their stuff, let me tell you that. Thousands of credits for a dwarf-made doll. Billions for a miniforge. Tens of billions for a plump helmet roast. And all they want in exchange is crap and raw materials. Let me tell you, they're a force to get rich on for a man willing to trade with them. Hey, it worked for me. Just be sure to bring them something they want, or you'll find out why the surface looks like it does.