What happens to the navmesh when mapped-out geometry disappears?

This is a pretty basic question, but I’m a bit confused about how the navmesh functions at runtime: assuming I have a single navmesh bounds volume covering the entire gameworld, and inside that volume I have levels being streamed in and out as the player moves around, what exactly happens to the navmesh? Does it recalculate itself every time something changes, or does it make a single, static mesh when the game begins, and retain that pathing data regardless of what happens inside the level?