If oddworld consists of one piece then it would be a dead planet. Becous of its lack of seismick activety no mountenous or high areas would be formed. Becous of erosion and lack of mounten formations would become compleatly flat.
So there has to be some tectonick schifting or other geological things happening on oddworld. But the gassious thing that could be plausible if the gass was condensed and heavy enoug, so the surface rock would have floated to the surface during formation of the planet. Other wise it would sink to the center of gravty like most of the iron on our planet went to the center when it was formd.
Or maibe it's something different all together. Something like anti gravety inbedded in the rock coutering the gravety of oddworld.
And iv just thought of another factor. The dencety of the air. If its verry dence and compressed it would counter gravety like water does. Becous that is dense and we are less dense or equaly dense we fall slower in water. And the odd worldians wouldent have a problem with the presure of this much denser air becous theyr adapted to it. Just like deep ocean fisch are adapted to the whater pressure there.
So more then enoug theorys to choos from, but wich one is true.
