Model in Sandbox completely dark

When I try to import my model in babylon.js sandbox, my model is completely dark, Here is a link to my model, you can try it yourself: - Google Drive

The sandbox can’t load the additional files related to your .obj (the .mtl and the texture files) because they are located in your local computer. You should host those files on a web site (like Github or any other) and use some code like this to display the scene:

var delayCreateScene = function () {
    var scene = new BABYLON.Scene(engine);

    // The first parameter can be used to specify which mesh to import. Here we import all meshes
    BABYLON.SceneLoader.Append("textures/test/", "Reservoir_multi-texture.obj", scene, function (newMeshes) {
        scene.activeCamera.attachControl(canvas, false);
        scene.activeCamera.alpha += Math.PI; // camera +180°

    return scene;

You will need to replace textures/test/ by the path to the directory of the web server where the files are located.

Alternatively, you can convert your .obj to a single .glb file and that will also work, because this file will contain all the needed textures and no external dependencies will be needed.