I recently saw this question asking about where the crimson or corruption was in their world, and the picture of their world that they included was the image below. Now I am not so familiar with the PC edition of the game as I am with the Xbox or mobile version - I only have about 25 hours played on Steam - so I never encountered some of the things on the map.

The first of these things is the sand coloured section of land under the normal desert. However, this is quite clearly some form of underground desert that is exclusive to PC. The confusion only arises upon looking at the somewhat, dark purple and light gray regions.

From what I can see, there appears to be minecart track linking some of these sections, so possibly some form of abandoned mine. But, upon searching for this on the internet, all I could find was this wiki entry about underground cabins. These are most definitely not what I am looking for, as these are wooden structures and are on a much smaller scale to the other coloured regions on the map.

Then, as I didn't know what else to search for, to find an answer to this question, I came here to ask!

So, what are the dark purple and light grey underground sections on the map?