How can i get the terrain vector below mouse.hit?

I have a puzzling question... From a players mouse.Hit, how can I find the coordinates of the closest terrain below?

For example, if a player is standing on a block and their mouse is pointing on that block, how can I get the coordinates, mostly the Y, of the terrain below?

Another example: If said player is also on a block, but in a cave, how can I get the terrain coordinates below, ignoring the terrain above?

So far i have been able to use mouse.Hit to get x and z, but still wondering how to get the Y from the terrain.

This has puzzled me for ages and any help would be greatly appreciated.