diff --git a/.gitignore b/.gitignore
index 3a672cb..c693edd 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,6 +16,13 @@
 # also local-only — see PmtilesBuilder.cached_extract)
 /data/geofabrik_cache/
 
+# Planetiler's own auxiliary-source cache and scratch working directory
+# (water polygons/Natural Earth/lake centerlines, --download; temp feature
+# DBs, --tmpdir) — relative to the app's CWD, which lands them here
+# alongside the rest of data/ (see extract_offline's --download comment)
+/data/sources/
+/data/tmp/
+
 # Offline Wikipedia ZIM for kiwix-serve (also large, also local-only —
 # same story, see docs/development.adoc)
 /data/kiwix/*
diff --git a/lib/maptool/pmtiles_builder.rb b/lib/maptool/pmtiles_builder.rb
index a403683..a4844fa 100644
--- a/lib/maptool/pmtiles_builder.rb
+++ b/lib/maptool/pmtiles_builder.rb
@@ -268,7 +268,15 @@ module Maptool
       # suffix made it fail with "Unsupported format tmp" instead of
       # actually tiling anything.
       tmp_path = File.join(data_dir, "offline.tmp.pmtiles")
-      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}")
+      # --download: the OpenMapTiles profile also needs a few small auxiliary
+      # sources (water polygons, Natural Earth, lake centerlines) alongside
+      # the OSM extract itself — without this flag Planetiler expects them
+      # to already exist on disk and fails instead of fetching them. They
+      # land under Planetiler's own default "data/sources/", relative to
+      # this process's working directory (the app root, so in practice
+      # that's data_dir's own "sources" subdirectory) — fetched once,
+      # reused by every later build.
+      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}", "--download")
       File.rename(tmp_path, File.join(data_dir, "offline.pmtiles"))
       File.write(offline_meta_path(data_dir), JSON.generate(sources: sources.map { |s| s.slice("code", "slug", "last_modified") }))
     ensure