Fractured object floating, instead of sinking

I followed this tutorial:

What I wanted was to break the landscape in the middle and let it sink. But it seems to be floating/flying away when the simulation runs. The landscape is a sculpted plane. And I increased the mass to 10000000 and the Collision Shape > Implicit Type = Box.

This setting makes the fractured pieces float/fly. See video below:

In the tutorial video, he does Collision Shape > Implicit Type = Level Set. But for my project, doing that results in no fracture.

Desired Output: The landscape forms a crack, breaks apart slowly and starts to sink/drop below.

How to achieve this output?