system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40,000] How bad are the Underhives?

Hello. My family lives in the Spires of this Hive City. My sister, tender-hearted and sensitive girl that she is, has gotten it into her silly little head that she and a few of her friends should head down into the Underhive- by themselves, with no protection! -to perform charity work for the people down there.

I've tried to talk her out of it, but she is adamant on going.

I'm sure that you people know just how awful it is down there. 

Please describe exactly what the Underhive is like, would you? 

Maybe hearing it from someone like you, someone who has been there, would knock some sense into her.
gpt
The Underhive is, in the most literal sense, hell. There is no electricity. There is no authority but who has the biggest gun. Life in the Underhive is a brutal, constant struggle for survival between mutant scum, gangs, and deep, nameless things that are born in darkness and live to devour. Average life expectancy of an unarmed newcomer in the Underhive is measured in minutes. A Spire dweller might not even make it that long.

Take it from one who knows, nothing good or wholesome dwells in the Underhive. It is a cesspool beyond redemption where the dregs of humanity and worse take root, and a good day is one where you retain a reasonable amount of blood. The only reason your sister would ever have to go there is as a member of the Adeptas Sororitas, to purge the darkness and filth with the cleansing flames of the Emperor's blessed pyrotheum.

She wants to do charity work, send her to the Lower Hive, where the generators are and the lights are on. There is hope for them.