diff --git a/imports/api/Common/id.js b/imports/api/Common/id.js
new file mode 100644
index 0000000..d419982
--- /dev/null
+++ b/imports/api/Common/id.js
@@ -0,0 +1,15 @@
+/*
+ * Fire/collection docs use `idGeneration: 'MONGO'`, so their `_id` is a
+ * `Mongo.ObjectID`, whose `.toString()` yields `ObjectID("<hex>")` — NOT the
+ * bare hex. Interpolating an `_id` straight into a URL or any string therefore
+ * produces `.../fire/archive/ObjectID("...")`, which the routes can't match.
+ *
+ * Use `hexId()` wherever an id becomes part of a string (URLs, comment
+ * reference ids, DOM ids). It returns the 24-char hex for an ObjectID and
+ * passes plain-string ids through unchanged. Do NOT use it for DDP method
+ * args whose `check()` expects `Meteor.Collection.ObjectID` — pass the object.
+ */
+export const hexId = id =>
+  (id && typeof id.toHexString === 'function' ? id.toHexString() : id);
+
+export default hexId;
diff --git a/imports/ui/components/Maps/MarkListeners.js b/imports/ui/components/Maps/MarkListeners.js
index 5a85b44..19ffc39 100644
--- a/imports/ui/components/Maps/MarkListeners.js
+++ b/imports/ui/components/Maps/MarkListeners.js
@@ -1,4 +1,7 @@
+import { hexId } from '/imports/api/Common/id';
+
 export const onMarkClick = (history, nasa, id) => {
   // console.log('onClick fired');
-  history.push(`/fire/${nasa ? 'active' : 'alert'}/${id}`);
+  // hexId: `id` is a Mongo.ObjectID whose toString() is `ObjectID("...")`.
+  history.push(`/fire/${nasa ? 'active' : 'alert'}/${hexId(id)}`);
 };
diff --git a/imports/ui/pages/Fires/Fires.js b/imports/ui/pages/Fires/Fires.js
index 4c79cf5..7790aeb 100644
--- a/imports/ui/pages/Fires/Fires.js
+++ b/imports/ui/pages/Fires/Fires.js
@@ -18,6 +18,7 @@ import FiresCollection from '/imports/api/Fires/Fires';
 import FireList from '/imports/ui/components/Maps/FireList';
 import FromNow from '/imports/ui/components/FromNow/FromNow';
 import { dateLongFormat, dateYYYYMMDD } from '/imports/api/Common/dates';
+import { hexId } from '/imports/api/Common/id';
 import CommentsBox from '/imports/ui/components/Comments/CommentsBox';
 import FalsePositiveTypes from '/imports/api/FalsePositives/FalsePositiveTypes';
 import FalsePositivesCollection, { falsePositivesRemap } from '/imports/api/FalsePositives/FalsePositives';
@@ -41,7 +42,7 @@ class Fire extends React.Component {
       // console.log(`Next when ${nextProps.when}`);
       if (nextProps.fire && (nextProps.alert || nextProps.active || nextProps.fromHash)) {
         // change url to archive with new _id
-        nextProps.history.replace(`/fire/archive/${nextProps.fire._id}`);
+        nextProps.history.replace(`/fire/archive/${hexId(nextProps.fire._id)}`);
       }
       this.setState({
         loading: nextProps.loading,
@@ -232,7 +233,7 @@ class Fire extends React.Component {
                 </ul>
               </div>
               <div className="comments-section">
-                <CommentsBox referenceId={`fire-${fire._id}`} />
+                <CommentsBox referenceId={`fire-${hexId(fire._id)}`} />
               </div>
          </Fragment>
         }