diff --git a/docs/architecture.adoc b/docs/architecture.adoc
index de1fcde..4fd8f07 100644
--- a/docs/architecture.adoc
+++ b/docs/architecture.adoc
@@ -154,6 +154,24 @@ manages in the page's own sidebar — both call the same
 `LayerToggleControl#toggle()`, so there's one source of truth for the
 panel's open/closed state either way.
 
+=== Debug inspector (Alt+click)
+
+Alt+click anywhere on the map opens a popup listing every rendered
+feature at that point — across all layers, not just this app's own GPX/
+FIT/radar layers — with its raw properties (`public/js/map.js`, a plain
+`map.on("click", ...)` handler gated on `event.originalEvent.altKey`,
+calling `map.queryRenderedFeatures(event.point)`). Inspired by NPMap4's
+`infoboxControl`, a similar generic "show me everything here" debug
+view kept separate from the curated popups those same features already
+have. Deliberately no dedicated UI for it — a keyboard-modified click
+needs no button, toggle state, or cursor change of its own, and it
+coexists fine with layer-specific click handlers (e.g. a GPX waypoint's
+normal popup) firing on the same click, since this is a power-user/
+debugging affordance, not a replacement for those. Feature properties
+can come straight from an uploaded GPX/FIT file (untrusted input) via
+this app's own layers, not just the base map, so every key/value is
+HTML-escaped the same way `gpx.js`'s popups already are.
+
 == Weather radar overlay
 
 `public/js/radar.js` adds a "Radar" map control that overlays recent
diff --git a/docs/nps-resources.adoc b/docs/nps-resources.adoc
index 51fa4e9..6609195 100644
--- a/docs/nps-resources.adoc
+++ b/docs/nps-resources.adoc
@@ -266,8 +266,16 @@ plugins (several still at `0.0.1-x`, pre-1.0), a few of which (`overview`,
 `carto`, `wms`) aren't even merged to `main` yet — see above. v5 is the
 direction NPS is migrating to, not (yet) a feature-for-feature
 replacement; expect this gap to close over time, not to stay this way.
-Two things worth noting from v4:
+Three things worth noting from v4:
 
+* *`infoboxControl`* — example:
+  https://www.nps.gov/lib/npmap.js/4.2.1/examples/basic/infobox.html. A
+  generic "click anything, show every raw property in a table" debug
+  view, kept separate from a dataset's own curated popup. *Adopted* — not
+  the code (Leaflet, not portable) but the concept: this app now has the
+  same thing as an Alt+click handler (see `docs/architecture.adoc`'s
+  "Debug inspector" section), showing `queryRenderedFeatures` results —
+  base map OSM tags included, not just this app's own layers.
 * *Geocoder control* — example:
   https://www.nps.gov/lib/npmap.js/4.2.1/examples/controls/geocoder-control.html.
   Config is `geocoderControl: { searchPlaces: true }`: a search box that
diff --git a/public/css/style.css b/public/css/style.css
index 3d3aa01..6451c32 100644
--- a/public/css/style.css
+++ b/public/css/style.css
@@ -438,6 +438,40 @@ html, body {
   margin: 4px 0 0;
 }
 
+.debug-popup .maplibregl-popup-content {
+  max-height: 320px;
+  overflow-y: auto;
+}
+
+.debug-popup__feature {
+  font-size: 11px;
+  font-family: monospace;
+}
+
+.debug-popup__feature + .debug-popup__feature {
+  margin-top: 8px;
+  padding-top: 8px;
+  border-top: 1px solid #eee;
+}
+
+.debug-popup__feature table {
+  border-collapse: collapse;
+  width: 100%;
+}
+
+.debug-popup__feature th {
+  text-align: left;
+  color: #888;
+  font-weight: normal;
+  padding: 1px 6px 1px 0;
+  vertical-align: top;
+  white-space: nowrap;
+}
+
+.debug-popup__feature td {
+  word-break: break-word;
+}
+
 .gpx-popup__time {
   color: #888;
   font-size: 12px;
diff --git a/public/js/map.js b/public/js/map.js
index cf9591b..8b2ceca 100644
--- a/public/js/map.js
+++ b/public/js/map.js
@@ -2,6 +2,7 @@ import {
   Map as MaplibreMap,
   NavigationControl,
   GeolocateControl,
+  Popup,
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
 import { RadarControl } from "/js/radar.js";
@@ -263,3 +264,40 @@ fetch("/api/offline-tiles")
     // is rather than guessing — this is the same-origin request itself
     // failing, not evidence offline tiles are actually unavailable.
   });
+
+// Alt+click anywhere shows every rendered feature at that point — base map
+// (OSM tags via OpenFreeMap/protomaps), GPX/FIT layers, radar, all of it —
+// with its raw properties, inspired by NPMap4's infoboxControl (a generic
+// "show me everything here" debug view, kept separate from the curated
+// popups those same features already have via their own layer-specific
+// click handlers, e.g. gpx.js's waypoint popups — both can fire on the same
+// click, which is fine, this is a power-user/debugging affordance, not
+// something replacing the normal popups). No dedicated UI for this
+// deliberately — a keyboard-modified click needs no button, state, or
+// cursor change of its own.
+map.on("click", (event) => {
+  if (!event.originalEvent.altKey) return;
+
+  const features = map.queryRenderedFeatures(event.point);
+  if (features.length === 0) return;
+
+  const html = features
+    .map((feature) => {
+      const rows = Object.entries(feature.properties)
+        .map(([key, value]) => `<tr><th>${escapeHtml(key)}</th><td>${escapeHtml(String(value))}</td></tr>`)
+        .join("");
+      return `<div class="debug-popup__feature"><strong>${escapeHtml(feature.layer.id)}</strong><table>${rows || "<tr><td>(no properties)</td></tr>"}</table></div>`;
+    })
+    .join("");
+
+  new Popup({ maxWidth: "320px", className: "debug-popup" }).setLngLat(event.lngLat).setHTML(html).addTo(map);
+});
+
+// Feature properties can come straight from an uploaded GPX/FIT file (an
+// untrusted source) via this app's own layers, not just the base map, so
+// every value is escaped the same way gpx.js's popups already do.
+function escapeHtml(text) {
+  const div = document.createElement("div");
+  div.textContent = text;
+  return div.innerHTML;
+}