tl;dr: Erin Catto, creator of Box2D, has released Box3D, an open-source 3D physics engine written in C17 with features like triangle mesh collision, continuous collision detection, SIMD contact solving, and cross-platform determinism. It originated as a fork of Valve physics programmer Dirk Gregorius's "Rubikon-Lite" (used in Half-Life: Alyx), then merged with Box2D v3.0 code, built to replace Unreal's Chaos physics for the game The Legend of California. It's already used in s&box, Esoterica, and Glenn Fiedler's 1000-player space game, though still considered alpha ahead of a v0.1 tag.