Weighty Problems

Prompted by a question from James Eck that boiled down to 'why do your players keep going back to Skull Mountain rather than setting up camp and staying there?'.

Here is the player-facing map of the Briars:

1 hex = 1 mile

\It is about 30 miles from the nearest town, Esber. The light yellow is the Lower Briars, the gray the High Briars, then badlands, then the plateau, then Skull Mountain. The line is the Old Road, a paved Roman-style road from the Old Times. On horseback with good weather a party can push from Esber to Skull Mountain in four and a half days.. The road is old, unmaintained, and in some placed washed out and turned to tracks (the biggest breaks are marked with yellow). The first break is called the Patrol Camp because it is the furthest any patrol will go. A wagon could potentially make it to the Patrol Camp in 3-4 days but past that nothing larger than a 2 wheel cart can go. In good weather a two-wheel cart could make it to Skull Mountain in 9 days.
BTW, in typical weather it takes six days and 20 days, respectively. In bad weather (snow on the ground and storms in Winter, say) it can take a month+.
Off the Old Road? No one has ever tried....

When parties set out to adventure in Skull Mountain they have to have enough supplies with them for the trip to and from plus 'loiter time', i.e., time in the dungeon. Sure, you could hunt along the way, but game animals shy away from the road and Skull Mountain and its massive flock of stirges has very few game animals within a 4 hour trip. This means 2 weeks of food per person and mount, minimum. For a real dungeon crawl you'll need a month of food per. With a person needing about a pound of preserved food a day, that is 30 lbs of provisions per person. Since forage along the route is effectively zero horses will need grain and some hay - about 5 lbs a day (minimum) or 150 lbs so a horse and rider really should carry 200 lbs of just food to Skull Mountain. This means the horses are moving slowly, so that gives you about 10 days in the mountain.
If you take a cart or two you can bring more, but extra travel time! Again, this leaves you 10, maybe 12, days of dungeon crawling. You can bring pack mules, for example, but they need their own food and if you have more than 1 or 2 you need teamsters and such.
In short, getting to Skull Mountain to adventure requires a supply chain.

The party has done this in the past. Almost every adventure to Skull Mountain has taken a few in-game days. Especially when they went to the Second Level they spent 10 days inside the mountain and left enough hidden provisions that the mapping expedition only needed to bring half of their estimated provisions, so it can be done. But until some way of providing food locally is found you will need to maintain routine pack trains along the Old Road at least 4 times a year at great expense just to have enough food.
As a result parties tend to plan ahead, arrive for a particular reason, spend 3-10 days inside the dungeon, and then retreat to civilization.

Guess what else is needed?
Light sources. The mapping expedition took an entire cart of just candles and lamp oil. That horse needed provisions, by the way.

This is why Skull Mountain has to be cleared out from time to time.
But that could be changing; the mapping expedition's base camp will stay at the mountain for at least 6 months and could remain as long as 2 years. The cost of the provisions is so high that any stay of more than a year means the expedition will be forced to rely upon treasure found while exploring to break even!

The players are wondering how Skull Mountain was ever permanently inhabited to begin with!