Display underground model

1.I tried to use clippingPlanes to crop the earth and place a ball inside the earth, but I can't observe the ball inside the earth by changing the radius of the ball.
2.I am using cesium 1.5.1 and win10.
3.this is my code:
    var position = Cesium.Cartesian3.fromRadians(0, 0, -6371004.0);
    var entity =viewer.entities.add({
              position : position,
              name : 'Red sphere with black outline',
              ellipsoid : {
                  //正球体
                  radii : new Cesium.Cartesian3(6271004.0, 6271004.0, 6271004.0),
                  material : Cesium.Color.RED,
                }
          });

          globe.depthTestAgainstTerrain = true;
          globe.clippingPlanes = new Cesium.ClippingPlaneCollection({
              planes : [
                  new Cesium.ClippingPlane(new Cesium.Cartesian3( 1.0, 0.0, 0.0), 0),
                  
                  new Cesium.ClippingPlane(new Cesium.Cartesian3( 0.0, -1.0, 0.0), 0)
              ],
              edgeWidth: edgeStylingEnabled ? 1.0 : 0.0,
              edgeColor: Cesium.Color.WHITE,
              enabled : clippingPlanesEnabled
          });
          viewer.trackedEntity = entity;

I think because it’s considered underground, it isn’t rendered. If you move the sphere up a little bit it starts to show.

Support for underground rendering has been a long-requested feature, see:

https://github.com/AnalyticalGraphicsInc/cesium/issues/5665

Definitely something we’d like to support in Cesium but isn’t something we’re actively working on yet.