FSX Terrein sinks after creating lake

Hello everybody.

I am very new to scenery design and this is my very first experience, so please be patient if my questions may look obvious to you.

I have installed FSX_KML and tried to follow up the brief tutorial about creating a lake.
I did try to build the "Idroscalo", a water strip near Milano Linate airport (Italy) that was used by Hydroplanes in the years '50.
I did draw ONLY the polygon of the water shape using Google Earth and built the BGLs, then I've placed them into the Addon Scenery\scenery folder.

This is the result I got:

Surrounding ground sinked "randomly" to zero feet level.
What was my mistake?
How can i solve this problem?
Thanks in advance.