My players have been operating out of a certain small town for my whole campaign. They've adventured around the area nearby quite a bit already (just finished Session 23).
Recently, my intended chapter boss has arisen nearby, in a particular location (call it The Dungeon) that I'd picked out before the campaign actually began. They have been close by to the location once, and it was the subject of a bunch of ominous rumor from NPCs early in the campaign. It has also been the subject of a series of increasingly obvious hints in the last few weeks: some NPCs known to the party went missing while exploring the area nearby; a monster army appeared and marched towards the town from The Dungeon; a hag that has become a vehicle for me to harass the PCs (and occasionally give vague plot guidance and hints) offered them a vision that included the entrance to The Dungeon; a townsperson who also saw the same vision later commented that they recognized the location; I updated the regional map to include markers on various locations the party has visited, including the area near The Dungeon.
The sooner that I can get them to The Dungeon, the more interesting I can make The Dungeon: if they wait too long, I'll have to make it a short single-session affair, because one of my players is leaving the party this summer. However the players still have not deduced that The Dungeon is where ... well, is where The Dungeon is. Their first action was to wander into the forest in search of the aforementioned hag. Now, they are making plans that include searching the nearby mountains, as well as dealing with other problems of the "wandering monsters" variety (a side effect of there being a Big Monster in a hole in the ground nearby). The Dungeon has not come up once in their discussions at the table.
On it's surface The Dungeon is just an ominous hole in the ground that they never actually explored, and I'm pretty certain that my players have completely forgotten about its existence. So, how do I remind them? Do I keep dropping hints in-universe? Should I address the players themselves, and tell them they're forgetting something? Any other suggestions?