fumaroles

“At one end of the vast C bitten from the castle a sin­gle great bastion-tower stood, almost intact, five kilometres high, and casting a kilometre-wide shadow across the rum­pled ground in front of the convoy. The walls had tumbled down around the tower, vanishing completely on one side and leaving only a ridge of fractured material barely five hundred metres high on the other. The plant-mass babilia, unique to the fastness and ubiquitous within it, coated all but the smoothest of vertical surfaces with tumescent hanging forests of lime-green, royal blue and pale, rusty orange; only the heights of scarred wall closest to the more actively venting fissures and fumaroles remained untouched by the tenacious vegetation.”
― Iain M. Banks, Feersum Endjinn