1. A concise explanation of the problem you're experiencing.
I have recently encountered a problem in Cesium. When I zoom the map, I often go under the map. How can I not get into the underground, the effect please look at the appendix
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
I want to improve user experience
4. The Cesium version you're using, your operating system and browser.
Cesium-1.4