And that's when they release the acid down the gutters in the sides of the corridor.Unless it's going incredibly fast, like, way more than 10ft/second, then they shouldn't need to save if the players actually say "we dodge to the side". If a player ums and ers about it, sure, an easy-ish save might be warranted, but the thing is 15' tall, so you barely even need to duck to avoid it, given it's a sphere, in a roughly square corridor, if you just move to the side, which, assuming the corridor is barely 15' wide and you're in the middle, takes under a second (just try walking 7.5'). If it's a total surprise and coming at level 30+ mph or something then a save might make sense, but 10'/second is 6.8mph.
This is kind of less of a trap and more of a feature.
The widest point would be at 7', which is over the heads of most PCs.
The thing about the 7.5 foot gap is that it's also infinitely thin. So would be crushed under the boulder, just between the boulder and the wall, which is not that much better. If you want even six inches of width, that reduces the height to under five feet of height, and if you want a whole foot of width you'll need a height of less than four feet. (I'd maybe be okay with a small creature managing it, especially since the only small creature in the party is the NPC kobold "guide."but the thing is 15' tall, so you barely even need to duck to avoid it
So my math is that volume of a sphere is 4/3 pi r^3 = 4/3/ pi 7.5^2 = 1767.15. I googled what a cubic foot of granite weighted and got 168 about pounds. 168 x 1767.15 =298,856.according to my bing search a 10 ft wide boulder would be are 21,875 pds but they depends on what stone it is.
It doesn't have to be, but I think it makes the most sense to be. (Though maybe there's something else fun inside.)Does the boulder have to be solid?
Have one of the PCs make an INT check to figure out exactly what you did.With that in mind what would you do?
The hollow rock is full of a magnetic liquid. Wile E. Coyote and an ACME Giant Magnet can drag the rock back up the slope. (Be careful about getting out after setting the hidden panel - especially if he is still carrying the magnet.)The concrete ball is hollow like a chocolate bunny.
That’s the only way this trap can get reset anyway.
Joking aside, the more you scale up the rock the more clearance there is to avoid it, unless you make the tunnel circular in cross section simply standing aside would avoid such a huge rock.