pub fn add_mesh(&mut self) -> DefaultBodyPartHandle {
        /*
        * Setup a random ground.
        */
        let quad = ncollide3d::procedural::quad(20.0, 20.0, 100, 100);
        let indices = quad
            .flat_indices()
            .chunks(3)
            .map(|is| Point3::new(is[0] as usize, is[2] as usize, is[1] as usize))
            .collect();

        let mut vertices = quad.coords;

        // ncollide generatse a quad with `z` as the normal.
        // so we switch z and y here and set a random altitude at each point.
        let mut x = -1.0;
        let len = vertices.len();
        for p in &mut vertices {
            x += 2.0 / len as f32;
            p.z = p.y;
            p.y = x;
        }

        let trimesh: TriMesh<f32> = TriMesh::new(vertices, indices, None);

        let shape =
            ShapeHandle::new(trimesh);

        let body = RigidBodyDesc::new()
            .set_translation(Vector3::new(0.0, 55.5, 0.0))
            .build();
        let body_handle = self.world.bodies.insert(body);

        let collider =
            ColliderDesc::new(shape).build(BodyPartHandle(body_handle, 0));

        self.world.colliders.insert(collider);

        BodyPartHandle(body_handle, 0)
    }