It is in Sternum, about 69 steps north of the spot the map marks with that name.

And 6 down from the surface once you are there.

• 1 — Sternum, as the map itself names it
• 2 — the ground above the treasure

• Key 3802: a locked door bars the way, and that is the number of the key that opens it.
• A rope. You drop in through a hole, and a rope is how you climb back out.
• Something for demon, giant spider and dwarf geomancer — they live down there.

The chest is 6 floors under the surface. Going down over it, this is what each floor uses:

1. Ground: down through a hole.
2. 1 down: down through a hole.
3. 2 down: down through a hole.
4. 3 down: down through a hole.
5. 4 down: down through a hole.
6. 5 down: down through a hole.

• 1 — the way in: a hole, ground
• → — the treasure lies this way

• 2 — a hole, 1 down
• → — the treasure lies this way

• 3 — a hole, 2 down
• → — the treasure lies this way

• 4 — a hole, 3 down
• → — the treasure lies this way

• 5 — a hole, 4 down
• → — the treasure lies this way

• 6 — a hole, 5 down
• → — the treasure lies this way

• 1 — the chest

The strongest 14 of 16 kinds whose nests reach this place.

One chest, one reward.