diff --git a/vendor/clouds3d.js b/vendor/clouds3d.js
index 0fe0d6f..0e89814 100644
--- a/vendor/clouds3d.js
+++ b/vendor/clouds3d.js
@@ -282,7 +282,9 @@ function init(canvas) {
 // 1 world unit = 1 tile at zoom 11 (~13 km). The shader maps
 // (hitPoint - uGroundCenter) / uGroundSize + 0.5 to UV, so the
 // mosaic centre always corresponds to UV (0.5, 0.5).
-var GROUND_TILE_URL = 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/';
+// Tile source: Carto basemaps Voyager style (with place names).
+// Free, CORS-enabled, OSM-derived. URL format {z}/{x}/{y}.png.
+var GROUND_TILE_URL = 'https://a.basemaps.cartocdn.com/voyager/';
 var GROUND_TILE_Z = 11;
 var GROUND_TILE_SIZE = 256;
 // Fetch direction: which way the tile corridor is oriented.
@@ -316,7 +318,8 @@ function fetchTileImage(tileX, tileY) {
         img.crossOrigin = 'anonymous';
         img.onload = function() { resolve(img); };
         img.onerror = function() { reject(new Error('tile fetch failed')); };
-        img.src = GROUND_TILE_URL + GROUND_TILE_Z + '/' + tileY + '/' + tileX;
+        // Carto/OSM-style format: {z}/{x}/{y}.png
+        img.src = GROUND_TILE_URL + GROUND_TILE_Z + '/' + tileX + '/' + tileY;
     });
 }