Howdy folks. I just had a fortress where I was lucky enough to find magma only 30z levels down. Whilst I could have just moved down to make my furnaces I decided to start attempting !!Science!! and built myself a pumpstack to keep all my industries up on the top 5 z levels. The stack was 24 zlevels high and followed the safe magma pumpstack 3x3 style that avoids lag. I made one alteration.
I added a floor hatch to every level so that I had a method of draining the stack for future modifications.
XXXXXXX XXXXXXX These are the two levels.
XX--oXX XXX---X X » Natural Wall
XX-X%XX XXX-H-X S » Up/Down Staircase (always in the same place)
XSWX%XX XXX---X W » constructed wall
XXX---X XSWX%XX % » Pump
XXX-H-X XX-X%XX o » Open Space (channelled tile)
XXX---X XX--oXX H » Hatch
XXXXXXX XXXXXXX - » Floor tile (dug tile)
This pump worked perfectly. Every single pump was created from iron metallic components (as they're the only technically magma safe metal and I had plenty of trees and iron ore) and stone blocks made from magma safe material. I shifted an entire 16x37 7/7 magma tiles up to the underneath of my furnace. Nothing broke, all lovely.
I then shut the flood gates leading to the magma sea, opened the specially designed flooding chamber and whammed open the floor hatches. I also shut off the power supply to the pumps. Everything looked fine. The pumps switched off, the hatches opened, the magma drained away peacefully (a few splashes into the lowest level stairway as the magma slightly overflowed that last open space... considering adding a door to prevent this).
I check back on the stack several months later and the entire stack of pumps has deconstructed. No fights, no monsters, no FBs, nothing. What could have caused it? I double checked every material, it was all magma safe and it didn't fall apart until AFTER the magma had evaporated.