diff --git a/lib/maptool/pmtiles_builder.rb b/lib/maptool/pmtiles_builder.rb
index b2090d8..d74fd94 100644
--- a/lib/maptool/pmtiles_builder.rb
+++ b/lib/maptool/pmtiles_builder.rb
@@ -66,10 +66,14 @@ module Maptool
 
     # fetch_tile: same injectable-seam reasoning as fetch_build_url above,
     # for the many individual Terrarium tile fetches this needs instead of
-    # one index lookup.
+    # one index lookup. This build can run for minutes with nothing else to
+    # show for it, so write_terrarium_mbtiles reports tiles_done/tiles_total
+    # into the status Hash as it goes — otherwise a poller sees "running"
+    # the whole time with no way to tell it apart from stuck.
     def self.build_terrain(bounds, data_dir:, fetch_tile: method(:fetch_terrarium_tile))
       begin_run!(TERRAIN_MUTEX, TERRAIN_STATUS)
-      Thread.new { run_job(TERRAIN_MUTEX, TERRAIN_STATUS) { build_terrain_pmtiles(bounds, data_dir, fetch_tile) } }
+      progress = ->(done, total) { TERRAIN_MUTEX.synchronize { TERRAIN_STATUS.merge!(tiles_done: done, tiles_total: total) } }
+      Thread.new { run_job(TERRAIN_MUTEX, TERRAIN_STATUS) { build_terrain_pmtiles(bounds, data_dir, fetch_tile, progress) } }
     end
 
     def self.begin_run!(mutex, status)
@@ -117,12 +121,12 @@ module Maptool
 
     # --- terrain.pmtiles ---
 
-    def self.build_terrain_pmtiles(bounds, data_dir, fetch_tile)
+    def self.build_terrain_pmtiles(bounds, data_dir, fetch_tile, progress)
       south, west, north, east = bounds
       tmp_mbtiles = File.join(Dir.tmpdir, "maptool-terrain-#{Process.pid}-#{Time.now.to_i}.mbtiles")
       tmp_path = File.join(data_dir, "terrain.pmtiles.tmp")
 
-      write_terrarium_mbtiles(tmp_mbtiles, south, west, north, east, fetch_tile)
+      write_terrarium_mbtiles(tmp_mbtiles, south, west, north, east, fetch_tile, progress)
       run_pmtiles!("convert", tmp_mbtiles, tmp_path)
       File.rename(tmp_path, File.join(data_dir, "terrain.pmtiles"))
     ensure
@@ -141,7 +145,7 @@ module Maptool
     # park-sized region, slow for a big one. Add a small bounded-concurrency
     # fetch pool if regenerating a large area turns out to matter in
     # practice.
-    def self.write_terrarium_mbtiles(path, south, west, north, east, fetch_tile)
+    def self.write_terrarium_mbtiles(path, south, west, north, east, fetch_tile, progress)
       db = SQLite3::Database.new(path)
       db.execute_batch(<<~SQL)
         CREATE TABLE metadata (name TEXT, value TEXT);
@@ -152,9 +156,19 @@ module Maptool
         db.execute("INSERT INTO metadata (name, value) VALUES (?, ?)", [name, value])
       end
 
-      (0..TERRAIN_MAX_ZOOM).each do |z|
-        tile_range(z, south, west, north, east).each do |x, y|
+      # tile_range per zoom computed once upfront (cheap — no fetching, just
+      # tile math) and reused below, so progress can report a real total
+      # from the very first call instead of only once the run finishes.
+      by_zoom = (0..TERRAIN_MAX_ZOOM).map { |z| [z, tile_range(z, south, west, north, east)] }
+      total = by_zoom.sum { |_z, tiles| tiles.size }
+      done = 0
+      progress.call(done, total)
+
+      by_zoom.each do |z, tiles|
+        tiles.each do |x, y|
           bytes = fetch_tile.call(z, x, y)
+          done += 1
+          progress.call(done, total)
           next unless bytes
 
           # MBTiles uses TMS row numbering (0 at the south) — the opposite
diff --git a/public/js/offline_data.js b/public/js/offline_data.js
index ab3cfb5..2fe4e8a 100644
--- a/public/js/offline_data.js
+++ b/public/js/offline_data.js
@@ -9,6 +9,24 @@ function formatBytes(bytes) {
   return mb >= 1024 ? `${(mb / 1024).toFixed(1)} GB` : `${mb.toFixed(1)} MB`;
 }
 
+function formatElapsed(ms) {
+  const totalSeconds = Math.max(0, Math.floor(ms / 1000));
+  const minutes = Math.floor(totalSeconds / 60);
+  const seconds = totalSeconds % 60;
+  return minutes > 0 ? `${minutes}m ${seconds}s` : `${seconds}s`;
+}
+
+// "Building…" alone gave no way to tell a slow build (terrain.pmtiles can
+// take minutes — lib/maptool/pmtiles_builder.rb fetches tiles one at a
+// time) from a stuck one. Elapsed time alone answers "is it alive" for
+// both files; tile counts (only terrain.pmtiles reports these) give an
+// actual sense of how much is left.
+function formatRunning(status) {
+  const elapsed = status.started_at ? ` (${formatElapsed(Date.now() - new Date(status.started_at).getTime())})` : "";
+  const tiles = status.tiles_total ? ` · ${status.tiles_done}/${status.tiles_total} tiles` : "";
+  return `Building…${elapsed}${tiles}`;
+}
+
 // One row of the Offline data panel — status text plus a "Regenerate for
 // current view" button, for either offline.pmtiles or terrain.pmtiles.
 // Both files behave identically from here (same status/build/build-status
@@ -103,6 +121,7 @@ class DataFileRow {
       }
 
       if (status.state === "running") {
+        this._statusEl.textContent = formatRunning(status);
         this._poll();
         return;
       }
diff --git a/test/test_pmtiles_builder.rb b/test/test_pmtiles_builder.rb
index 5cde21f..fda9dab 100644
--- a/test/test_pmtiles_builder.rb
+++ b/test/test_pmtiles_builder.rb
@@ -113,7 +113,34 @@ class PmtilesBuilderTest < Minitest::Test
     thread.join
 
     assert_path_exists File.join(@data_dir, "terrain.pmtiles")
-    assert_equal "done", Maptool::PmtilesBuilder.terrain_status[:state]
+    status = Maptool::PmtilesBuilder.terrain_status
+    assert_equal "done", status[:state]
+    assert_operator status[:tiles_total], :>, 0
+    assert_equal status[:tiles_total], status[:tiles_done]
+  end
+
+  # fetch_tile blocks after the very first call so the "running" progress
+  # can be observed mid-flight, deterministically, rather than only ever
+  # seeing the final done/total once the (very fast, faked) build finishes.
+  def test_build_terrain_reports_progress_while_running
+    ENV["PMTILES_BIN"] = @pmtiles_ok
+    gate = Queue.new
+    first_call = true
+    fetch_tile = lambda do |_z, _x, _y|
+      gate.pop if first_call
+      first_call = false
+      "bytes"
+    end
+    thread = Maptool::PmtilesBuilder.build_terrain([48.140, 17.100, 48.141, 17.101], data_dir: @data_dir, fetch_tile: fetch_tile)
+
+    20.times { break if Maptool::PmtilesBuilder.terrain_status[:tiles_total]; sleep 0.01 }
+    status = Maptool::PmtilesBuilder.terrain_status
+    assert_equal "running", status[:state]
+    assert_operator status[:tiles_total], :>, 0
+    assert_equal 0, status[:tiles_done]
+  ensure
+    gate.push(nil)
+    thread&.join
   end
 
   def test_build_terrain_still_succeeds_when_some_tiles_are_missing