diff --git a/code/map_data_render.js b/code/map_data_render.js
index 38f1e7ef..212c285d 100644
--- a/code/map_data_render.js
+++ b/code/map_data_render.js
@@ -5,6 +5,14 @@
 
 window.Render = function() {
 
+  // when there are lots of portals close together, we only add some of them to the map
+  // the idea is to keep the impression of the dense set of portals, without rendering them all
+  this.CLUSTER_SIZE = L.Browser.mobile ? 10 : 4;  // the map is divided into squares of this size in pixels for clustering purposes. mobile uses larger markers, so therefore larger clustering areas
+  this.CLUSTER_PORTAL_LIMIT = 4; // no more than this many portals are drawn in each cluster square
+
+
+  this.entityVisibilityZoom = undefined;
+
   this.portalMarkerScale = undefined;
 }
 
@@ -393,28 +401,113 @@ window.Render.prototype.createLinkEntity = function(ent,faked) {
 
 
 window.Render.prototype.updateEntityVisibility = function() {
+  if (this.entityVisibilityZoom === undefined || this.entityVisibilityZoom != map.getZoom()) {
+    this.entityVisibilityZoom = map.getZoom();
 
-  if (this.portalMarkerScale === undefined || this.portalMarkerScale != portalMarkerScale()) {
-    this.portalMarkerScale = portalMarkerScale();
+    this.resetPortalClusters();
 
-    console.log('Render: map zoom '+map.getZoom()+' changes portal scale to '+portalMarkerScale()+' - redrawing all portals');
+    if (this.portalMarkerScale === undefined || this.portalMarkerScale != portalMarkerScale()) {
+      this.portalMarkerScale = portalMarkerScale();
 
-    //NOTE: we're not calling this because it resets highlights - we're calling it as it resets the style (inc size) of all portal markers
-    resetHighlightedPortals();
+      console.log('Render: map zoom '+map.getZoom()+' changes portal scale to '+portalMarkerScale()+' - redrawing all portals');
+
+      //NOTE: we're not calling this because it resets highlights - we're calling it as it resets the style (inc size) of all portal markers
+      resetHighlightedPortals();
+    }
   }
 }
 
 
 
+// portal clustering functionality
+
+window.Render.prototype.resetPortalClusters = function() {
+
+  this.portalClusters = {};
+
+  // first, place the portals into the clusters
+  for (var pguid in window.portals) {
+    var p = window.portals[pguid];
+    var cid = this.getPortalClusterID(p);
+
+    if (!(cid in this.portalClusters)) this.portalClusters[cid] = [];
+
+    this.portalClusters[cid].push(pguid);
+  }
+
+  // now, for each cluster, sort by some arbitrary data (the level+guid will do), and display the first CLUSTER_PORTAL_LIMIT
+  for (var cid in this.portalClusters) {
+    var c = this.portalClusters[cid];
+
+    c.sort(function(a,b) {
+      var ka = (8-portals[a].options.level)+a;
+      var kb = (8-portals[b].options.level)+b;
+      if (ka<kb) return -1;
+      else if (ka>kb) return 1;
+      else return 0;
+    });
+
+    for (var i=0; i<c.length; i++) {
+      var guid = c[i];
+      var p = window.portals[guid];
+      var layerGroup = portalsFactionLayers[parseInt(p.options.level)][p.options.team];
+      if (i<this.CLUSTER_PORTAL_LIMIT || p.options.guid == selectedPortal || artifact.isInterestingPortal(p.options.guid)) {
+        if (!layerGroup.hasLayer(p)) {
+          layerGroup.addLayer(p);
+        }
+      } else {
+        if (layerGroup.hasLayer(p)) {
+          layerGroup.removeLayer(p);
+        }
+      }
+    }
+  }
+
+}
+
 // add the portal to the visible map layer unless we pass the cluster limits
 window.Render.prototype.addPortalToMapLayer = function(portal) {
-  portalsFactionLayers[parseInt(portal.options.level)][portal.options.team].addLayer(portal);
+
+  var cid = this.getPortalClusterID(portal);
+
+  if (!(cid in this.portalClusters)) this.portalClusters[cid] = [];
+
+  this.portalClusters[cid].push(portal.options.guid);
+
+  // now, at this point, we could match the above re-cluster code - sorting, and adding/removing as necessary
+  // however, it won't make a lot of visible difference compared to just pushing to the end of the list, then
+  // adding to the visible layer if the list is below the limit
+  if (this.portalClusters[cid].length < this.CLUSTER_PORTAL_LIMIT || portal.options.guid == selectedPortal || artifact.isInterestingPortal(portal.options.guid)) {
+    portalsFactionLayers[parseInt(portal.options.level)][portal.options.team].addLayer(portal);
+  }
 }
 
 window.Render.prototype.removePortalFromMapLayer = function(portal) {
 
   //remove it from the portalsLevels layer
   portalsFactionLayers[parseInt(portal.options.level)][portal.options.team].removeLayer(portal);
+
+  // and ensure there's no mention of the portal in the cluster list
+  var cid = this.getPortalClusterID(portal);
+
+  if (cid in this.portalClusters) {
+    var index = this.portalClusters[cid].indexOf(portal.options.guid);
+    if (index >= 0) {
+      this.portalClusters[cid].splice(index,1);
+      // FIXME? if this portal was in on the screen (in the first 10), and we still have 10+ portals, add the new 10to to the screen?
+    }
+  }
+}
+
+window.Render.prototype.getPortalClusterID = function(portal) {
+  // project the lat/lng into absolute map pixels
+  var z = map.getZoom();
+
+  var point = map.project(portal.getLatLng(), z);
+
+  var clusterpoint = point.divideBy(this.CLUSTER_SIZE).round();
+
+  return z+":"+clusterpoint.x+":"+clusterpoint.y;
 }