The levels for the DLC Tunnels of Terror can be found in Ridden Hives.
These hives are scattered around the normal levels, I recommend replaying level 1 of act 1 since it is short and there is a big change there is a hive towards the end of the stage.

Also the level The Nursery always seems to be the second level when you do deeper into the hive nest. unlike the other 6 stages which are random when you enter the hive.

Left Ventricle - Secret in The Nursery:
https://youtu.be/TluMRoMUU90

Nook, or Cranny? - Secret in Sunken Passages.
https://youtu.be/VpFvsMw77lQ

Unholy Grail - Secret in The Cut.
https://youtu.be/GquVolR0SkY

Precarious Perch - Secret in Brood Lair.
https://youtu.be/ZzosxUwe5Ww

Backtrack - Secret in 300 Below.
https://youtu.be/JIJ2UgqlbL0

Round the Riverbend - Secret in Blood Stream.
https://youtu.be/pGHDT2GFR5E

Pipe Dream - Secret in Caustic Cesspool.
https://youtu.be/Bcjq-KRvIbc

I also have a other guide for the main game, give this a look if you are interested in finding the golden skulls there.
https://gtm.you1.cn/sharedfiles/filedetails/?id=2623432626

Having trouble finding the golden skull in act 5 (Children of the Worm DLC)?
https://gtm.you1.cn/sharedfiles/filedetails/?id=2856485636

Golden Skull location in ACT 6 (River of Blood) :
https://gtm.you1.cn/sharedfiles/filedetails/?id=2898435283

Thank you for checking out my guide.