Direct answer
Objectives follow journal display order. Some depend on your choices; follow the active objective in the game.
Result
While in the passageways underneath Novigrad (I've been there as well, and don't recommend them as tourist destinations - if the stench doesn't get you, the monsters will) Geralt met a scholar who was excavating in search of traces of long-lost cultures. The professor asked Geralt for help removing the lid from a certain ancient sarcophagus.