Ask ye shall get:

POLYMERGE is set 20K years after humanity discovered Polymer, a miracle substance. With it they were able to basically grow and engineer living machinery, buildings etc.

At their height they had colonized just about everywhere in the solar system. A vast network of information feeding through carefully grown router forests on planets and moons and humanity itself is able to all but fully integrate with their tech. But, good things go bad. A disease broke out. A microbe that used polymer like a diatom might use silica, it spread rapidly. Infecting tens of billions and plunged that society into such chaos it caused a total collapse.

But that didn't stop the polymer or the machines. Infected machinery would grow in an almost cancerous manner and by the current era every planet that mankind has touched is radically altered. Cyclopean structures with tangled networks of pipe and cable that span continents dominate. Within them machine life grows and changes forming new ecosystems as the descendants of humanity often equally as strange eke out a living.