ROAM
Builds terrain as a binary triangle tree and splits or merges triangles every frame against a screen-error metric — fine-grained but CPU-heavy.
ROAM — Real-time Optimally Adapting Meshes — was presented by Duchaineau et al. in 1997 (Lawrence Livermore). The terrain is a binary triangle tree: where detail is needed a triangle splits in two, where it is not two merge back. Two priority queues, one for splits and one for merges, let the mesh adjust incrementally and stay frame-coherent — only what must change between two frames does.
The result is extremely fine-grained: dense triangles near the camera, sparse ones at the horizon. But the bookkeeping runs on the CPU, one triangle at a time, and once graphics cards grew fast it paid off more to hand the GPU large static blocks. Geomipmapping and chunked LOD took over for that reason.