Neat lore bard or druid combo:

transmute rock 

then cast campfire/bonfire cantrip  on a restrained victim for 3d8

start of there turn another 3d8

end of turn another 3d8

so 9d8 potential per round.

This and mold earth for some easy pretty decent homes

Quote from RetailPleb >>

This spell specifically says it transforms mud/quicksand into soft stone. Why the distinguishing adjective 'soft' in this case? It's not defined anywhere, and so doesn't seem to have any mechanical consequences what kind of stone the spell makes.

 Compared to hard stone, like from the Wall of Stone spell where each panel has AC 15 and 30 hit points per inch of thickness.  This soft stone is AC 15 and 25 hit points.

This affects a 40ft cube, does that mean if you target the ground with the upper side of the cube, that the lower side of the cube (40ft underground) will mean that things will sink 40ft down?
Or is it disregarding "complete" sinking saying that the mud is thick enough to traverse across without sinking all the way (potentially just 5ft down), hence the 4ft per 1ft of movement penalty?
Also at what rate would sinking occur?

On a side note why would you waste a dispell turning it to rock when the restraint factor is already built into the mud already and would require repeat saves. 

Quote from Galion_Lightbringer >>

Do people feel that the requirement of having mud, quicksand or rock under targets is pretty limiting here? Or would most DM's rule that pretty much any natural ground is composed of some combination of these?

 It could be, depending on the area.  Dry dirt or normal sand isn't any of those things.  So farmland, forest, desert, beach, for example could make the spell ineffective.

I'm curious about something.

The duration for this spell is "until dispelled".  Which would indicate that, once dispelled, the affected substance would return back to the original substance it was before the spell was cast.

So, if you cast rock to mud, on say a hard rock surface, such as a stone floor, and had enemies sink in, and then dispelled the effect, it would turn it back to the original hard rock surface correct? Not the type of soft stone surface mentioned in the "Transmute Mud to Rock" part of the spell?

What would happen to creatures trapped in the mud when it is dispelled and turned back to its original stone state? Would they make a dex save like in the "Transmute Mud to Rock" section? If it's not soft stone, would they be trapped, unable to break free? Would they have whatever part of their bodies stuck in the mud crushed? Would they die? Or is this something that would be left to the DM to determine on a case by case basis?

It you used it on rock that contained something of interest, like gemstones or crystals, would they be turned to mud as well? Or would they be more easily extracted/mined?

If you are a Scribes Wizard, you could change the damage type of the falling stone from 'bludgeoning' to 'fire' (as long as you have a 5th level fire damage spell) - should be easy to convince the DM that it's lava at that point.

The theoretical use of this spell on a stone bridge over a chasm while enemies are on it.

The worldbuilding implications of this spell are pretty big. This is essentially concrete, the spell.

This plus Spike Growth is devestating for the enemy. You can only move 5 feet a turn, take damage for it, and need a save each turn. 

Combine with moonbeam and enjoy - especially with Large/Huge/gargantuan creatures. Telekinetic feat also useful with this combo.

Quote from varg1 >>

Quote from LeorioNailo >>

So could you transform the walls of a dungeon, or better yet a tower, into mud collapsing the entire structure?

 If you want a collapse, you'd only have to transmute part of the rock under the foundation. (Think war mining & petards.)

Quote from Galion_Lightbringer >>

Do people feel that the requirement of having mud, quicksand or rock under targets is pretty limiting here? Or would most DM's rule that pretty much any natural ground is composed of some combination of these?

 That strikes me as meaning it'll work everywhere but desert, so no undermining the tents of Bedouins. :D (Or their camels. ;p )

 I have allowed Sand to Mud in the Al-Qadim setting I'm running, mostly as an obstacle to movement.

"Nonmagical rock of any sort" RAW it should work on ice?

Quote from stevenphilpott1 >>

"Nonmagical rock of any sort" RAW it should work on ice?

 I think DND follows the 4 primordial elements, earth, air, fire, water.  As such I'd expect it only applies to the earth element.

RAW with modern physics yes it would apply and make slushies.

Does "choose an area of stone or mud that you can see that fits within a 40-foot cube" also mean, you could use an area of 160foot by 10foot? 

I read from the rules, that it would only need to fit into such a 40-foot-cube, but can have any dimension still falling into this max size.

rework this with some fire damage and you could turn the mud to lava.

Question: If someone is held in Force Cage, could another caster Cast Transmute Earth on the ground below. Turning it to mud. The Force Cage is pushed into the mud and then the mud changed back to rock?

Force Cage being concentration spell, is stopped. 

Buried alive?!

I am right with this?

Quote from TalthorTheMeatiness >>

Question: If someone is held in Force Cage, could another caster Cast Transmute Earth on the ground below. Turning it to mud. The Force Cage is pushed into the mud and then the mud changed back to rock?

Force Cage being concentration spell, is stopped. 

Buried alive?!

I am right with this?

No. Forcecage specifically says that the "cage" it creates is immobile. It can't be pushed (into mud or anywhere else).

Quote from roarswhisper >>

Just dispelling the spell would turn the mud back into rock, you don't need the second spell slot. Also, it specifically says 'non-magical' so the second spell wouldn't work anyway.

 Please remind me how you would dispel a spell without a spellslot.