system
Given the following post, respond as if you're commenting on Reddit:
human
[LotR] What was the purpose of the Endless Stair in Moria?

As far as we know, there's nothing down there except darkness, a flooded underground cavern, and nameless ancient monsters that gnaw at the roots of the earth. Why would the Dwarves shift tens of thousands of tons of rock and dig straight down to reach *that*?
gpt
Moria was the best source of *Moria silver*, *true silver*, or as the elves call it, *mithril*. 

You need to keep going deeper to find more true silver. It's a little different than other fine metals, and it's veins tend to be far deeper than wide. We all learn this in Mining 101 and the topic is fleshed out in Minerals 305, check your textbooks or ask a masterminer.