How to recognize some parts of the terrain?

Question

I created an island and want to check when I reach the island.
How can I add a tag to the island? The island built from a set height of the terrain. I added a picture for better understanding:

Answer 1

1. You can bound your island using invisible cubes with colliders as triggers.
2. You can detect a terrain texture and check if a current texture equals an island's texture - detecting a terrain texture is at a following link: detecting terrain texture