Estenreth's topography

As the shadowlands' northeast geopolitics go, the deep elves need only the western Marches of Caetel. Normally elves would leave far-flung caverns like the Estenreth vault to some other friendly race, like they don't contest the dwarves for Rockhome's underworld. Here at Estenreth, the elves did once have a problem with the local neighbours, not being always friendly. But even in 1000 AC when those neighbours are at least grudgingly quiet, the elves maintain their hold here.

The elves' reason for holding this cavern must, then, be that they have use for the cavern's resources.

The Bethis mine in the Dessaris Crests coincides with an Orcs of Thar "Lava Flow" river, from a magma lake above the Jorfyn Depression (that lake also flows out to Naralf's Hole, which feeds the Jorfyn). Underground, "hills" and "mountains" imply topographical shifts, up or down; "badlands" implies jagged terrain, chasms, and outcrops. So we aren't told if the Bethis mine be below the river, or above it.

So let's make a decision: ashore. The Dessaris Crest overlooks both the Grunelgard and the lake, which are at about the same level. The lava is in the Dessaris, as in, in: under its surface. The mine connects to the lava flow.

That lava then flows beneath the Grunelgard too, east of that. I have marked a red circle on this map, where that lava runs through a maelstrom down to Mystara's magma core. Just southeast of the Grunelgard.

The mine isn't a soul-crystal hotspot, but it's still quite useful. The Lava Flow burps up all manner of minerals from that big magma lake up above. It's also a wonderfully hygenic garbage-disposal.