ToME WikiContents
Heart of the Earth
The entrance to the Heart of the Earth (purple > in ascii) can be found on level 25 of Mirkwood.
The Heart of the Earth is a strange and potentially annoying place. The walls are in constant movement. The algorithm is not the Game of Life, but something else vaguely similar. The arrangement begins as totally random but soon develops fluctuating, but relatively stable and large open spaces surrounded by "frothy" stone. Chain summoners would have a field day, but luckily the Heart of the Earth is too shallow for that to be a problem. Watch out for mobs with distance attacks in the open spaces, though.
Good digging ability is not absolutely necessary, since any wall on your path will eventually move away, but digging helps your sanity. A mage's probability travel is very effective but naturally one must remember to detect monsters to avoid bumping into a pack of them. Conversely, stone prison is useless since the walls you create won't keep in place. The layout of the entire dungeon is visible to you at once, which is nice.
The Heart of the Earth is guarded by Golgarach, the Living Rock. It will drop a randart, the getting of which is usually the only reason to visit.
Chatter
ReenenLaurie: I visited yesterday this dungeon for the first time, and though not very exciting (not lots of monsters) it was kinda awe inspiring to think I'm in the heart of the earth, and to see how it moves about. I got submerged in the atmosphere, and though the dungeon is one where you dive, kill the boss and exit, everyone should visit this dungeon at least once in their ToME careers. (Not with each character though)
Anonymous: This doesn't say what level range the dungeon is. I was expecting a single map (for some reason or another 
-- is it always HoE 25-36?
RogerBarnett: Yes it is, and Golgarach is always on the bottom level. It's certainly worth a visit if you're quite high level and feel like a randart. I've had a very spiffy ring from him before 
NerdanelVampire: Heart of the Earth is very deep in Annals of Ea. It isn't a terribly difficult dungeon except if chainsummoning gets out of control which it easily does if you don't kill stuff quickly enough. If a sky drake gets to summon more sky drakes it becomes pretty much impossible to get rid of the level's ancient dragons without genociding them. Stone Prison is very temporary. Trees get deleted by rock. The true beauty of the dungeon generating code just doesn't show up at the depths ToME gives it.
ZiggyG: Has anyone else noticed a lot of stat potions in this dungeon, I just dove through it in about 15 min with a LVL 30 sorceror and found 6 stat potions, all strenth, int, and chr.
Spamtrap: summon unmovable pet is a blessing here. It does not get deleted, you can clean up almost all the dungeons with your colorful m-s.
Kevlar Moneyclips: Do you need to do anything special to find the stairs down into The Heart of the Earth? because I've gone down to 33 on Mirkwood several times and haven't been able to find a purple down stairs.
Sirrocco: There will always be at least one (perhaps only one?) purple down-stairs on any given Mirkwood level 25, but it's easy to miss if you aren't looking for it specifically.