Not yet a reality, but often seriously discussed, is terraforming `aitering an extracterrestrial body to make it capable of supporting  Earth's life forms.
( Not ( ( ( ( ( yet ( a reality ) ) , ) ( ( but ( often seriously ) ) discussed ) ) , ) ( ( is ( ( terraforming ` ) ( ( aitering ( an ( extracterrestrial body ) ) ) ( to ( make ( it ( capable ( of ( supporting ( ( Earth 's ) ( life forms ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (RB Not) (NP (NP (RB yet) (DT a) (NN reality)) (, ,) (VP (PRN (CC but) (ADVP (RB often)) (ADVP (RB seriously))) (VBN discussed)) (, ,)) (VP (VBZ is) (VP (VBG terraforming) (`` `) (S (VP (VBG aitering) (NP (DT an) (JJ extracterrestrial) (NN body)) (S (VP (TO to) (VP (VB make) (S (NP (PRP it)) (ADJP (JJ capable) (PP (IN of) (S (VP (VBG supporting) (NP (NP (NNP Earth) (POS 's)) (NN life) (NNS forms)))))))))))))) (. .)))
Terraforming exists but is never discussed, it is rather taboo. 
( ( Terraforming ( ( exists but ) ( is ( never discussed ) ) ) ) ( , ( it ( ( is ( rather taboo ) ) . ) ) ) )
(ROOT (S (S (NP (NNP Terraforming)) (VP (VP (VBZ exists)) (CC but) (VP (VBZ is) (VP (ADVP (RB never)) (VBN discussed))))) (, ,) (NP (PRP it)) (VP (VBZ is) (ADJP (RB rather) (JJ taboo))) (. .)))