Unknown substance is a hardcoded material that takes the form of a light gray liquid. It freezes at 10000 °U and boils at 10180 °U , and has the same density as water. It does not seem to appear in this version of the game, but it was well-known in v0.28.181.40d for appearing in glowing pits occupied by tentacle demons, along with both types of filth.
[MATERIAL:UNKNOWN_SUBSTANCE] - reconstructed from data extracted from memory
[STATE_NAME_ADJ:ALL_SOLID:unknown frozen substance]
[STATE_NAME_ADJ:GAS:unknown boiling substance]
[STATE_NAME_ADJ:SOLID_POWDER:unknown frozen powder]