Sector 3 / Pyrosphere

Pyrosphere is the super-hot, Lava-filled area of the Bottle Ship. Enemies in this area include Dessgeega, ULF 28 and ULF 39. Apparently, some areas of this sector are so hot that the Varia Suit is needed to access them.

The name comes from a recent press release by Nintendo of Europe, which also confirms the name of Cryosphere. 

Another location named "Volcano Island" can be seen on a computer monitor in the recent developer interview video. It is unknown if this refers to the Pyrosphere or another area.