The Polar Chasm is a nether biome covered in Hardened Ice and Snow. The sky has a blue tint compared to the other Nether biomes. There is no vegetation and glowstone deposits are replaced with regular ice. Ice and hot spring water pools can also spawn here.
It used to be the rarest biome in the mod, being so hard to find that the creator Forstride implied that they made it so rare so nobody would find it.
In 1.7.10 and earlier, mushrooms spawned in Polar Chasm biomes and only snow covered the biome. Since it was so rare, many people thought it was removed in 1.7.10 and that it would not return in 1.11.2. The old biome ID was 58.
The biome was readded in 1.11.12, but appears to be significantly more common.
Older (1.7.10 and below)Edit