The Stalactite layer is made of blocks of Stalactite, Siltstone, patches of Saltrock, Mud, Mold and sometimes Glowing Mushrooms. Not many caves are leading down here, but still a number of connected larger caves traverse this layer.
The Stalactite layer is mostly dark except for the faint illumination of glowing mushrooms and the green-yellow light of creatures when they touch Mineral water.
You can meet Mirus with faintly glowing golden eyes and immobile Warmworms spawning on the Stalactite layer, sometimes also Night Rocksters and rarely Night Pigsies that have ventured downwards from further up.
Also walking on the hot rocks of the Lava layer will make a heat-scale show up that will fill itself with red color for as long as you venture in this hot environment, until your player-character will start to burn if no counter-measures are applied in time.
To mine Stalactite, Siltstone, Salt Rock, Mold, Glowing Mushrooms and Tourmaline you need at least an Obsidian Mining Cell.
The Stalactite layer can be mined with the help of Super Excavators (that will make holes of up to 7x7x7 blocks and collect half of the blocks it has removed), but not with any other Excavators (currently, as of R26 in December 2015). Even the Super Excavator is not able to mine the even harder rocks that the Lava layer is made of though.