diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index a56036e..d54abd6 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -387,3 +387,45 @@ help: the values span -32743..32633, close to the full range, with no
 per-position structure (533, 371, 401 and 187 distinct at positions 0-3).
 Recorded as a characterisation, with its range, so the next attempt starts
 from the composition rather than re-deriving it.
+
+### Sky sub-object 0 is a slowly-varying sequence, not independent records
+
+**Range measured:** level 0, sky region `+365760 .. +388880`, 23,120 bytes.
+
+**Method:** Pearson correlation between the byte sequence and itself at
+increasing lag, against a shuffled control. This measures whether consecutive
+records are *related*, which the position-wise distinct-value counts could
+not show.
+
+**Result:**
+
+| lag (bytes) | correlation |
+|---|---|
+| 1 | +0.050 |
+| 4 | +0.560 |
+| **8** | **+0.746** |
+| 16 | +0.666 |
+| 32 | +0.602 |
+| 64 | +0.432 |
+| 128 | +0.140 |
+| 256 | −0.044 |
+| **512** | **+0.304** |
+| 1024 | +0.247 |
+| shuffled control (lag 1) | +0.003 |
+
+The profile has a peak at lag 8, decays to a zero crossing near 256, and then
+a second, smaller peak at 512 before decaying again.
+
+This is what settles the earlier "no constant substructure" reading. The
+records do not share fixed fields, but they are *not independent*: consecutive
+8-byte records are strongly related, and that relation repeats at 512 bytes,
+i.e. every 64 records.
+
+Slowly varying values, related within a record, repeating every 64 records, is
+the shape of a keyframe or lookup table -- which is what one would expect to
+find in sky data, where a colour or fog parameter is sampled rather than
+computed per vertex.
+
+**Still not decoded.** Nothing here names the values or the sampling. But the
+structure is now characterised well enough that a decode attempt has a shape
+to aim at rather than a field to hunt for.