My dorfs took the risk and decided to dig out a cotton candy vein. The legendary miners quickly dug out everything above the magma sea without finding it hollow, and then proceeded down through it into the part encrusted in semi-molten rock. And that's when they made a terrible mistake: by digging the candy around the ramps they used to go down, the ramps were destroyed. They did this thrice before they realized they were trapping themselves.
So now they're stuck in a vein of cotton candy, surrounded by semi-molten rock, three levels below the magma sea. I got them to build a ramp out of cotton candy, but when look at the ramp it is shown as "Unusable Rough Raw Cotton Candy Block Upward Slope", and the dorfs don't use it to go up. They have no other materials available to them.
How can I rescue them?