diff --git a/docs/plan/level-assets.md b/docs/plan/level-assets.md
index 23b679c..28673e0 100644
--- a/docs/plan/level-assets.md
+++ b/docs/plan/level-assets.md
@@ -730,3 +730,71 @@ A tfrag can now be turned into positioned, UV-mapped, textured, lit triangles,
 told which LOD to draw, and morphed toward its parents at the right rate.
 Everything needed for a first frame is parsed, and the frame is verified by
 screenshot rather than by assertion.
+
+## Core index field order (2026-10-11)
+
+Repeatedly re-deriving these by hand wasted real time this session, so the
+order is written out. It matches `LevelCoreHeader::parse` in
+`crates/gc-formats/src/level_core.rs`:
+
+| offset | field |
+|--------|-------|
+| 0x00   | gs_ram count |
+| 0x04   | gs_ram offset |
+| 0x08   | tfrags |
+| 0x0c   | occlusion |
+| 0x10   | sky |
+| 0x14   | collision |
+| 0x18   | moby_classes count |
+| 0x1c   | moby_classes offset |
+| 0x20   | tie_classes count |
+| 0x24   | tie_classes offset |
+| 0x28   | shrub_classes count |
+| 0x2c   | shrub_classes offset |
+
+Note `gs_ram` and the three class tables are **count/offset pairs**, so every
+one of them is two words. The region offsets (`tfrags` through `collision`) are
+single words. Counting the class tables as one word each shifts every field
+after them, which is how `collision` got read as `sky` and produced a page of
+implausible collision headers on all 27 levels.
+
+## Sky (2026-10-11)
+
+Region is `[sky, collision)` and is well defined, but the header is **not**
+yet decoded and the layout is not uniform.
+
+Reading `u16` at the head, most levels give:
+
+```
++0x00 0
++0x02 0
++0x04 4 or 6      varies per level
++0x06 0
++0x08 0           level 2 is the exception
++0x0a 3..6        looks like a layer count
++0x0c 0
++0x0e 64 (0x40)
++0x10 128 or 192  varies
++0x12 64 (0x40)
+```
+
+Level 2 breaks the pattern: `+0x08` is 0x0109 and the three words after the
+apparent count are shifted and different (`80 00 00 00 00 01 00 00 40 00`).
+So a single fixed header does not fit every level, and the count at `+0x0a`
+cannot be trusted to be one.
+
+A table of offsets follows at `+0x20` for the layers. The offsets ascend and
+sit in the last 8-15% of the region, but the count of *non-zero* offsets is
+sometimes less than the declared count (level 26 declares 6, has 4 real
+offsets and 2 zero slots). Unresolved.
+
+## Moby geometry (2026-10-11)
+
+Class entries are 0x20 bytes in `core_index.bin`, fields
+`geom_offset, o_class, unknown_8, unknown_c, textures[16]`; all 238 level-0
+moby classes carry `textures == ffff..ff`, i.e. no texture slots.
+
+`geom_offset` points into `core_data`, and the spans tile without overlap. The
+data at that offset does not start with a recognisable header -- it opens with
+zero words and then packed half-float VIF data (`55 55 15 3e` shows up 4 bytes
+in). Not decoded.