Middle right side of the screenshot, to the right and below the steam vent: a geyser placement in neutronium "biome". The geyser hasn't been placed, just the rock surrounding it.
This map was generated in first release version, but I didn't see the bug until now.
In output log, the world seed is listed as:
Custom Settings: World=worlds/Oasis,WorldgenSeed=1638552134
Generate the map from the seed? I'm not sure how to "translate" seed from log...