Even before the third game came packaged with an editor, the Marathon series had a small but intense mod-scene. The seal of the .MAPs was Rubicon, released in 2001 — six years after its parent, Marathon Infinity.
I’m not sure whether Rubicon was good, but it was fascinating and effective. It was (is, thanks to Aleph One) simultaneously more ambitious than any other mod — crammed with original maps and bespoke assets — and more faithful to the atmosphere of Marathon Infinity (if not the earlier games).
Rubicon eats itself. It knots and blossoms. Infinity‘s austere architectural convolutions are driven to overwhelming, fractal excess. One of the levels is a remake of a Marathon 2 level that was itself a remake of a level from Marathon, at this point dissolving into some kind of psychedelic baroque while killing you over and over. The textures and environmental sprites are vastly richer than anything in the original games, attempting (rightly or wrongly) to show Marathon‘s universe in much more detail than ever before; clunking machines and living alien architecture. It almost doesn’t matter that the mad fluidity of vanilla Marathon play frequently collapses in exhaustion trying to keep up, or that the mod’s story is torn apart by conflicting directions.
Most remarkably, this is a mod for a relatively obscure game that was already long in the tooth when it was released. Out of love or stubbornness, the creators persisted.
I’ll leave you with someone playing the infamous Hex Level 73, the old-fashioned maze level as an object of insufferable beauty.