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Transmute Rock to Sand, and Transmute Rock to Mud are both fourth level spells that have a caveat stating they can only affect up to a depth of 10 ft from the surface they are cast on. Does this need to be a flat, 10-ft square on one side of the cube to be affected, or can it take another shape? Likewise, could the surface exist within the cube to be affected? Such as targeting both walls of a one foot passageway to take 5 ft of stone off the left side and 4 ft off the right side?

The specific use case I was imagining would be to have a trained badger, wearing adamantine claw extenders (or an effigy of a badger with adamantine claws) dig a pilot hole that is at least 1.6 ft wide. Because a circle with a radius of 1.6 ft has a circumference of 10 ft, I was wondering if it would be possible to cast the spell on the stone the pilot hole has been dug through, affecting the rest of the 10-ft cube of stone.

The end goal would be to use this fourth level spell to facilitate carving large passages through stony earth with relative ease. Other issues such as sapping, dealing with water sources that could flood the entire tunnel as soon as the stone is transmuted, and the inevitable random encounters, can all be disregarded for this question. What I'm most concerned about is the viability of using the spell in this fashion.

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  • \$\begingroup\$ I'm curious, why badger? It's maybe 10x smaller in both width and height than a normal person, but its weapons will be 1000x lighter. Would it really have an easier time digging an equivalent tunnel than a half-orc barbarian with an adamantine greataxe? Or is it the price of a small adamantine claw extender compared to a larger weapon? \$\endgroup\$
    – Phoenices
    Oct 18, 2022 at 1:22
  • \$\begingroup\$ (Tangentially, how will you get the 150ish pounds of stone per foot out? The badger sounds like it would be a bit... hampered by the stone-slicing claw extenders.) \$\endgroup\$
    – Phoenices
    Oct 18, 2022 at 1:24
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    \$\begingroup\$ @Phoenices The badger would have an easier time digging a tunnel for itself than would a half-orc barbarian with an adamantine greataxe as the badger has a burrow speed. However, I think what the asker really wants is a dire badger that can also burrow and specifically leaves a tunnel when it does. \$\endgroup\$
    – Hey I Can Chan
    Oct 18, 2022 at 2:45
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    \$\begingroup\$ Well that is unfortunate. I wasn't the impression that a regular badger was equivalent to a small size dire badger with fewer hit dice. Given that it lacks the permanent tunnel aspect of a dire badger, it seems like a significantly poorer choice. Of course, a dire badger leaves a 5 ft hole, which makes it less of an issue whether you can use the spell in this fashion. \$\endgroup\$
    – Benjamin
    Oct 18, 2022 at 3:43
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    \$\begingroup\$ Doesn't stone cease to be unworked as soon as you dig through it? \$\endgroup\$
    – annoying imp
    Oct 20, 2022 at 16:49

2 Answers 2

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No, it does not require a flat surface

The text regarding the transformation aspect for the spell Transmute Rock to Mud is:

This spell turns natural, uncut or unworked rock of any sort into an equal volume of mud. Magical stone is not affected by the spell. The depth of the mud created cannot exceed 10 feet. (...) If transmute rock to mud is cast upon the ceiling of a cavern or tunnel, the mud falls to the floor and spreads out in a pool at a depth of 5 feet.

There is no demand that the surface is flat, or contiguous, only that the stone within the volume is natural, uncut or unworked, and that the depth from the surface cannot exceed 10 feet.

If you had an appropriate burrowing creature that could burrow through solid rock and would leave a tunnel in its wake , you could use the spell to turn rock around it into mud.

Both badgers and dire badgers cannot dig through solid rock (and only dire badgers leave a tunnel behind, credit to HeyIcanChan), and the spell unfortuntely states that it turns rock into mud, not earth. This then leads to the question if rocky earth would count as rock, which probably would be be up to your DM's interpretation.

Assuming it would, if you read the last sentence in the spell quote, any tunnel with a diameter of up to 5 feet would be filled entirely with mud in the area. (I'm not sure how that works out from a conservation of matter perspective, but similar issues exist if you target a 10-foot cube of ceiling and it only creates a 5-foot deep pool of mud in the same area. This may require some DM adjucation to make sense.)

Given that, and given the fact that you will need some way to remove the mud to be able to create a passage through rock, that seems to be very cumbersome. It would appear simpler to just use the 5-foot tunnel the dire badger leaves behind to begin with.

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  • \$\begingroup\$ it was my understanding that the text "if transmute rock to mud is cast upon the ceiling of a cavern or tunnel, the mud falls to the floor and spreads out in a pool at a depth of 5 feet" was intended to mean that if the cube of mud falls onto a flat surface it will spread out until it fills its container or reaches a depth of five feet. I believe it is a statement of how thick the mud is and how it doesn't disperse like water. This tracks based on how adding brush on top of it is enough to make it load bearing. \$\endgroup\$
    – Benjamin
    Oct 18, 2022 at 21:04
  • \$\begingroup\$ @Benjamin Yes there are several ways to interpret how the mud spreads - maybe another question. As it's written any of them will fill the tunnel up to 5 feet, so would fill a tunnel that is just 5 feet in diameter. You could conclude that when its not cast only at the ceiling, it should be less. All of this is only somewhat related to your question though, as you say you don't really care about that part. \$\endgroup\$
    – Groody the Hobgoblin
    Oct 18, 2022 at 21:18
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The spell Transmute Rock to Mud has an area entry of: Up to two 10-ft. cubes/level (S)

(S) Shapeable If an Area or Effect entry ends with "(S)," you can shape the spell. A shaped effect or area can have no dimension smaller than 10 feet. Many effects or areas are given as cubes to make it easy to model irregular shapes. Three-dimensional volumes are most often needed to define aerial or underwater effects and areas.

(SRD – Spell Descriptions – Aiming a Spell - Areas)(emphasis mine)

So, technically, if you want to affect the walls, the ceiling and the floor of a tunnel, they all must measure at least 10 feet – otherwise the spell does not work. Bad news for the badger...

Surfaces of natural, uncut or unworked rock are, of course, never really flat or straight, let alone right-angled but the game mechanics translate every natural area into squares or cubes in order to calculate dimensions. Thus, a round tunnel always becomes a "cubic" one to make things playable.

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