How do I setup a "reset position" type plane?

My goal right now is to have a sort of safeguard against falling through the world and ending up in the endless void that exists below the scene’s terrain. Optimally, what I’d like is a floor underneath the existing terrain that, should the player fall through the world, would touch the plane and have their position reset to some space above the ground to prevent falling through the world. I’m sure there’s something like this available in regards to scripts but I haven’t found one yet. Any help would be appreciated!

EITHER

(a) create an empty object with a trigger box collider under your geometry that is big enough in X and Z to be udner all of it and do the reset in an OnTriggerEnter method

OR

(b) Put a check of your Y position in the player Update method and do the reset if it gets below a trigger value.