diff --git a/docs/re/INDEX.md b/docs/re/INDEX.md
index c5e1cd48..d1a0f0d4 100644
--- a/docs/re/INDEX.md
+++ b/docs/re/INDEX.md
@@ -101,6 +101,7 @@ files, which is how the same ground got covered twice.
 | [`structures/isl-message-dialogue-link.md`](structures/isl-message-dialogue-link.md) | Mission scripts as dialogue — built-in 64 -> message id -> caption text | ✅ CONFIRMED total, 2 683/2 683 call sites across all 28 stages resolve, no residue |
 | [`structures/mission-objective-counter.md`](structures/mission-objective-counter.md) | `REMAINING OB` — the mission's own objective counter, in RAM |  ✅ CONFIRMED for one Stage 02 run: a big-endian u32 whose value |
 | [`structures/movie-subtitles.md`](structures/movie-subtitles.md) | Movie subtitles & the movie ↔ mission ↔ text chain | — |
+| [`structures/mcol-collision.md`](structures/mcol-collision.md) | `MCOL` — same container and map parameters as `REGN` | 🟡 OPENED — `POF0` at `data_size+16` 11/11, bbox pad words 11/11, `extent == max−min` 11/11, and bbox + cell-size distributions identical to `REGN` (2/6/3 and 2/9); everything past `0x40` ❔ |
 | [`structures/regn-map-grid.md`](structures/regn-map-grid.md) | `REGN` — a per-map spatial grid (and `MCOL` beside it) | ✅ CONFIRMED for the header, which self-checks on all 11 objects on |
 | [`structures/savegame-format.md`](structures/savegame-format.md) | Save file (`savedata`) — container ✅ exact, 3 fields named ✅, rest ❔ (2026-08-11) | ✅ CONFIRMED for the container and the chunk layout — parsed off the |
 | [`structures/sound-slb.md`](structures/sound-slb.md) | Sound bank audio — `sound.pak` / `.slb` / XMA1 | — |
diff --git a/docs/re/structures/mcol-collision.md b/docs/re/structures/mcol-collision.md
new file mode 100644
index 00000000..e9102f04
--- /dev/null
+++ b/docs/re/structures/mcol-collision.md
@@ -0,0 +1,59 @@
+# `MCOL` — the same container as `REGN`, and the same map parameters
+
+**🟡 Opened 2026-08-26.** `MCOL` sits beside `REGN` in `hidden/MiscBin.pak`, 11
+of each, and has never been decoded. This page establishes what it shares with
+`REGN` — which is a lot, and gives the next attempt a large head start.
+
+## ✅ Same container
+
+Over all **11** objects:
+
+| | |
+|---|---|
+| `POF0` fixup table at `data_size@+4` + 16 | **11 / 11** |
+| bbox pad words are `1.0` / `1.0` / `0.0` at `+0x1C`, `+0x2C`, `+0x3C` | **11 / 11** |
+| `extent == max − min` for the `0x30` block | **11 / 11** |
+
+So the header prefix is byte-for-byte the same shape as `REGN`'s: magic, data
+size, then bbox min / bbox max / extent as `f32[4]`, then a triple at `0x40`.
+The `POF0` mechanism applies, which means **the `chunk + 0x10` base and the
+loader's own pointer list are available here too** — the two things that cracked
+`REGN`.
+
+## ✅ And the same map parameters, exactly
+
+The bounding boxes and the `0x40` triple are not merely similar — the
+distributions are **identical**:
+
+| | `MCOL` | `REGN` |
+|---|---|---|
+| bbox ±250 000 | 2 | 2 |
+| bbox ±50 000 | 6 | 6 |
+| bbox ±25 000 | 3 | 3 |
+| `0x40` = 50 000 | 2 | 2 |
+| `0x40` = 10 000 | 9 | 9 |
+
+Eleven maps, and for each one an `MCOL` and a `REGN` describing the same volume
+at the same cell size. `0x40` is the **cell size** in `REGN`; the same values in
+the same multiplicities here is strong evidence it is the cell size in `MCOL`
+too — though note this is a match of *distributions*, not a demonstrated
+object-to-object pairing, which would need the two linked by name or by a stage's
+tables.
+
+## ❔ What is not yet known
+
+* **Everything past `0x40`.** `MCOL`'s words at `0x50`–`0x84` do **not** look
+  like `REGN`'s (`REGN` has grid dims at `0x50`, six `u16` counts at `0x60` and
+  six section pointers at `0x70`; `MCOL` has a large value, two mid-range values
+  and `112` at `0x50`, mostly zeros at `0x60`, and `0x05050501` at `0x70`). The
+  headers agree on the spatial prefix and diverge after it.
+* **My `POF0` delta decoder is wrong here** — it emits eight leading zero deltas
+  before plausible offsets, so either the stream has a preamble I am not
+  skipping or my 6/14/22-bit reading is off. The working decoder is
+  `tools/re-capture/regn_decode.py` on branch `auto/regn-reader`; **use that
+  rather than re-deriving it**, which is the mistake I made here.
+
+**First step for whoever continues**: run the known-good `POF0` reader over an
+`MCOL`, list which words are relocated, and read the header slots those pointers
+occupy. That is exactly the move that turned `REGN` from twenty failed
+correlations into a decode.