Alien Sector

You have decided to travel to Green 23 in the Alien Sector.  This is several levels below ground, where the corridors are lit by a strange light reminiscent of sodium, sulphur or methane.  You come to a doorway that is sealed.  It has a symbol displayed on it representing a breather mask.  This reminds you that you are about to enter an alternative atmosphere zone, so you may need an oxygen breather mask...