Fallen Angels, by Niven, Pournelle & Flynn.

High above the northern hemisphere the scoopship's hull began to sing. The cabin was a sounding box for vibrations far
below the threshold of hearing. Alex MacLeod could feel his bones singing in sympathy.

Piranha was kissing high atmosphere.

Planet Earth was shrouded in pearl white. There was no break anywhere. There were mountain ranges of fluff, looming cliffs, vast plains that stretched to a far distant convex horizon. An illusion, a geography of vapors as insubstantial as the dreams of youth. If he were to set foot upon them...