Why, when I try to make a hole in the terrain, does the texture just disappear in that place?

Question

Unity

I'm new to unity, so I can't understand what I'm doing wrong, because in the tutorials, a deepening quietly appears, and my texture disappears

Answer 1

Raise the entire Terrain on creation

Answer 2

Terrarian is the texture (heights), in what sense to make a hole? Do you need a hole in the map that you can fall through?