Hundreds of storeys below ground that is a huge railway system that passes through the tectonic plates.
My Mother and me have a small workplace where we sort glassware. Bottles, jars, bowls, wineglass, vases... everything made of glass. It all comes tumbling down to us through the hundreds of layers of railway systems.
We work in a symphony of glas on metal notes.
It something shatters not too far from us it is of great value. Like a treasure. I often climb through the system to get to these precious shards. Some of the rails are similar to those in Gringotts in Harry Potter, consisting of two round pipe like metal rails.
We work all day but it is peaceful, almost meditative to sort all the glas into it's proper category. Once sorted we place them into see through cartons and stamp them so they can be brought below for building materials. They are building a giant library with glass walls. Each carton becomes a brick in its walls. Light will filter through and when the underground wind comes they will tinkle like a small waterfall.