An Underlying Problem is a music from the Shovel Knight soundtrack. This is the theme of the Lost City in Shovel of Hope and Plague of Shadows. It is Music Sheet #31 in Shovel of Hope. It can also be listened to in Sound Test after completing either of these campaigns once.
Music Sheet #31 can be found in the Lost City, in a Treasure Chest of a room where Charflounders must be bounced on with Shovel Drops. It can be accessed from the second checkpoint's room, by jumping across explosive blocks.
"An excavation project of this scope would be respectable, in happier times."
- "Bard: Let's see here... Oh, you found "An Underlying Problem"! This driving ditty was meant to increase productivity. But it backfired completely - everyone started dancing!"