Colin Marston's studio (which he was forced to close a couple of years ago) was called Menegroth the Thousand Caves. I knew it was a Tolkien reference, and he records a lot of metal, so I assumed it was some orc fortress.
I just looked it up, and it turns out to have been an elf city.
Menegroth was a wholly underground city accessed only through a stone bridge over Esgalduin. While the Dwarves did the major work, the Elves also participated in the construction: the pillars imitated beeches with branches and leaves, the halls were lit by golden lanterns and there were silver fountains, and many figurines of animals. It is told that nightingales and other birds lived within the caves. Thus Thingol had the most beautiful palace a king ever had owned east of the Sea.[4]
Surprising. I always thought it was weird that the elves of Mirkwood lived underground, unlike every other elf state. It turns out, there was precedent, and it wasn't something Tolkien thought was out of character for them.