I've been touring underground tunnels looking for a short-term rental. This is the room that I've decided to rent. The room includes an incredibly dilapidated twin bed and a wooden desk. To give you an idea of the size of this particular room, it's as big as a large walk-in American closet and smaller than a queen sized bed. These rooms cost 350RMB/month. Some rooms have a double-bed or a TV, and those cost a bit more.
Each room comes with an ethernet cable; internet costs 60RMB/month.
There is no way to tell if it is daytime or nighttime as there are no windows in the room. There are two ways to find out the time of day: look at the time on your cellphone/clock or walk all the way up into the upper-ground world.