diff --git a/src/cesiumViewer.js b/src/cesiumViewer.js
index 45b84b0..c029105 100644
--- a/src/cesiumViewer.js
+++ b/src/cesiumViewer.js
@@ -27,6 +27,14 @@ export function createCesiumViewer(containerId) {
   return viewer
 }
 
+/**
+* Loads country borders from a GeoJSON file and adds them to the Cesium viewer.
+* The data source is stored as viewer._countryBorders for visibility management.
+*
+* @param {Cesium.Viewer} viewer - The Cesium viewer instance
+* @returns {Promise<Cesium.GeoJsonDataSource>} The loaded GeoJSON data source
+* @throws {Error} If the GeoJSON file cannot be loaded or parsed
+*/
 export async function loadCountryBorders(viewer) {
   const dataSource = await Cesium.GeoJsonDataSource.load(
     '/data/geo/world_simplified.geojson',