diff --git a/docs/architecture.adoc b/docs/architecture.adoc
index 72b118c..22cd22a 100644
--- a/docs/architecture.adoc
+++ b/docs/architecture.adoc
@@ -103,6 +103,18 @@ No PMTiles file ships with the repository — it is expected to be
 supplied locally (see `docs/development.adoc`) and stays gitignored
 because such extracts are typically large binary files.
 
+== Map permalink
+
+The map's `hash: "map"` option (`public/js/map.js`) turns on MapLibre
+GL JS's built-in `Hash` control — nothing hand-rolled here. It keeps
+`#map=<zoom>/<lat>/<lon>` in the URL in sync with the current view (via
+`history.replaceState`, so panning never spams the back button) and,
+if the page loads with one already in the URL, opens there instead of
+the default `CENTER`/`ZOOM` — the same permalink pattern OSM and Google
+Maps use, so copying the URL bar shares the exact spot someone's
+looking at, and reloading the page doesn't reset the view back to the
+default.
+
 == Weather radar overlay
 
 `public/js/radar.js` adds a "Radar" map control that overlays recent
diff --git a/public/js/map.js b/public/js/map.js
index 1a55f90..3ba1723 100644
--- a/public/js/map.js
+++ b/public/js/map.js
@@ -108,6 +108,12 @@ const map = new MaplibreMap({
   style: ONLINE_STYLE,
   center: CENTER,
   zoom: ZOOM,
+  // MapLibre's built-in Hash control: keeps #map=zoom/lat/lon in the URL in
+  // sync with the view (via history.replaceState, so panning doesn't spam
+  // the back button) and, if the page loads with one already in the URL,
+  // opens there instead of the default CENTER/ZOOM — the same "grab the URL
+  // to share this exact spot" permalink pattern OSM/Google Maps use.
+  hash: "map",
 });
 
 map.addControl(new NavigationControl(), "bottom-right");