Using Direct X compute shaders to emit and update up to 5,000,000 particles simultaneously. Particle-Mesh collision detection is done using a pre-baked octree.
To use simply arrange the mesh as you wish and click Bake Octree. For maximum tree depth it may take between 1-2 minutes to complete.
Collision accuracy really depends on number of nodes and tree depth, but the octree creation can take extremely long beyond a depth of 5.