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If I understand you correctly, you mean the chasms in the dungeons, right?
There are special objects in the SubterraObjects section: Wall_dno_down, dno_down, and the rest of this type. If they don't show up in the editor, you can find them in the DungeonWalls section of the AdvMapStaticShared tab.

Прокрастинирую...
"...Завтра станет лучше
но солнце так и не взошло..."