If you go to the lake near Silden you are able to run on the ground of the lake. To do this you have to go to the smallest place on the map before the river begins (right of the small rock, you can look at the village and the sunken castle). You should come from the way from Geldern to Silden, and before going left or right you easily walk straight to the lake(to this rock exactly), but you have to sprint a few meters before "entering" the lake. If you make it at the right place you can walk on the ground of this lake and see the castle and other destroyed houses. After you entered the lake you don't have to sprint anymore and you don't have to breathe.