As the rods heat up, their zirconium cladding can rupture, which releases gaseous radioactive iodine inside, and may even cause the uranium to melt and release much more radioactive material.

A large planet, like Jupiter or Neptune, that is composed largely of gaseous material.

Gaseous elements in urban underground can transfer or spread to the surface of the ground, and different surface has different capability of ventilation due to its covering material.