I've had this glitch literally since 2006 but I thought that maybe UC wouldve fixed it (nope). When I delete a lot in neighbourhood view, the terrain fully flattens out. I had a lot sloping down to the coast that when deleted, turned the prime beachfront real estate into a straight cliff with a sharp drop off into the sea. It flattens it from whatever the road level is at.
This has happened pretty much every single time I've deleted a lot since way back when sims 2 was just a base game but I've never really bothered to try and fix it until now (14 years too late luv)
If anyone knows of any software/mods that would allow neighbourhood terrain manipulation inside the game, I don't have Simcity 4 plus I'm already halfway done filling out this n'hood. I would v much appreciate any/all links.
Thank you!

There is a cheat (modifyneighborhoodterrain on/off), you can see its instructions here.

Oh thank you! This is really helpful <3

Hey so this helped-ish. The terrain im trying to modify is in water but when I flatten it, new water doesn't appear in its place and instead is replaced with the ~dark void~

It's this neighbourhood specifically , the islands in the three corners look weird with my skybox so I wanna get rid of them.

Try exiting the neighbourhood and then re-entering; the water is calculated afresh when you load the hood.