v0.31:Hidden fun stuff

Reaching Hell

On digging sufficiently deep (Usually z-level ~40) you will eventually find huge magma lakes. These are surrounded by semi molten rock, which can not be cleared by digging but seems otherwise stable. Eventually, you will reach a layer which contains nothing but molten rock and little to no possibilities to pass. You can reach the critical layer by digging through solid rock which gets rarer the deeper you dig. Beyond, there are two possibilities.
1. Digging through an adamantine vein: Adamantine veins are shaped like tubes and breach through any layer of molten rock, right down into hell. Some of those veins are hollow by default and have each layer separated through floor tiles.
2. Entering a demonic fortress: These fortresses are found at most once per map, most likely instead of the former glowing pits. They also go right through to hell, however they are defended by undead creatures and demons on the lower levels.

Demonic Fortresses

A demonic fortress is found usually once per area containing sufficient mountain tiles. It is located almost at the bottom, once you discover it it will be announced. They contain random undead and skeletal creatures, and hordes of those. Note that they are connected through slate walls with hell, so demons spawning there can not immediately cross into the fortress and swarm you. All in all, those structures seem to be the only reasonable way to enter hell itself, as there is plenty of space inside to set up traps and defense.

Hell itself

Like the demonic fortress, you will receive an announcement on reaching hell. ("Horrifying screams come from the darkness below"). Once you discover hell, tons and tons of demons will be spawned. Prepare for the worst, there can easily be hundreds of demons spawned. It is unclear whether the amount of demons is finite.