In basically the worst possible place to deal fire damage fireball gets worse? Even in the abyss, where most things are resistant to fire, fireball is almost as good. A 20 foot radius is 40 feet across, a way bigger area than a 20 foot across cube. If you can deal 4d6 to 8 enemies or 3d12 to 4, it's not so different. I mean you could argue that erupting earth is better against a few, close together fire resistant targets. BUT when there's only a few enemies, AoE damage becomes less valuable in general. There are plenty of times where you need a fireball and no substitutes will do, there are very few situations where erupting earth will swing a battle so dramatically (though you might get lucky and get three 12s)

Quote from ThelonFairblade >>

20' cube vs. 20' radius (fireball).

3d12 (19)  vs.  8d6 (28) at 3rd.  4d12 (26)  vs.  9d6 (31) at 4th.  5d12 (32)  vs.  10d6 (35) at 5th.  6d12 (39)  vs.  11d6 (38) at 6th.  Takes 3 upcast levels to catch up to Fireball.

 To be fair, that still hasn't even caught up since a 20 ft radius is 40 ft across, much bigger than a 20 ft cube.

Druids can't learn Fireball, though.

I wonder, what happens to a building if you casting it inside. Will the floor rise 20 ft. What will Happen to people in a room, when they can not escape?

I have a party member wanting to cast this on a cave roof.. while the spell describes casting it on the ground, the discussion is that technically the roof is still the same material as the ground.this would be the same if the cast it on the side of a cliff

what everyone’s thoughts on this and should there be extra damage from the falling debris? I was thinking 1d6 per 10 feet would be appropriate to a max of 12d6.  
Keen to hear everyone’s thoughts on this

Quote from Haddanis >>

I have a party member wanting to cast this on a cave roof.. while the spell describes casting it on the ground, the discussion is that technically the roof is still the same material as the ground.this would be the same if the cast it on the side of a cliff

what everyone’s thoughts on this and should there be extra damage from the falling debris? I was thinking 1d6 per 10 feet would be appropriate to a max of 12d6.  
Keen to hear everyone’s thoughts on this

 I would rule against it, as the spell specifically states “on the ground” and doesn’t make any mention of the surface material being a relevant factor. I like the creativity of your players, but that’s beyond the written scope of the spell, and you can start getting into squirrelly territory if you apply the “well technically” mindset too much.

Quote from Neal_Gatton >>

For Dungeon of The Mad Mage, it says in the adventure that you cannot shape the stonework of the dungeon with magic. In theory then, this spell should not do anything and that is how I ruled it initially. However I am going to change it so that instead of turning up the earth, the caster magically creates the earth for a moment, allowing the damage. It still doesn't create rough terrain but will still allow for some serious damage. Just an option for any DMs like me who don't want spells to be useless in Under Mountain.

 I think this means you cant stone shape but it doesn't mean you cant cast erupting earth.