Underground roads cost diamonds suddenly.

[young sphinx]

I started a new map based off of my fantasy book, but when I tried to connect the kingdoms with an underground highway, it made me pay a large amount of diamonds. I checked my other regions and this doesn't happen for them, it just takes cash. There's an option to pay real life money to unlock it, but I've never paid actual money on this game before, so how did I unlock it in my other regions? Is there a way I can do this with my fantasy Kingdom?

[rapid pebble]

You can build a road authority which costs diamonds

[young sphinx]

Ooh, okay. I'll give it a try. Thank you