How to load meshes when you are inside an area

I have for example an underground dungeon, but I only want it to load when you are inside of it… gow do i make it so that LOD groups still work, but for them only to load when you are inside that cube/area

To make my myself more clear… how do i make a cylinder, and only when i am inside of it will it start loading the assets

this sounds more like a level streaming situation to me.its the same thing that other games do when they put the player into a elevator or enclosed space so that the game can load the next level.