A gas meant to be breathed during the deepest part of a technical dive. Usually carried in tanks mounted on back and therefore often called also the back gas. Requirements from a bottom gas are:
If the gas contains only oxygen and nitroxen and the oxygen content is more than 21%, it's called nitrox. If it also contains helium, it is called trimix. If the trimix has less than 15% oxygen it cannot be breathed at the surface and is called hypoxic, otherwise it is called normoxic.
Since the human body needs a partial pressure of 0.15 bar of oxygen instead of a percentage, gas mixes with very low percentage of oxygen become safely breathable when deep enough. When using these hypoxic gas mixes it becomes necessary to breath a separate travel gas until at depth where the bottom gas becomes safe to breath.