[Unity 5]es possible to create an open world in a hollow world

Hi,

I am an amateur game designer and I 'm stuck in the terrain editor .

I would create an open world in a reverse earth. Of course I do not want everyone to be open, but given the sensation of beings in the player .

Exemple


I know I 'm going to create the illusion with a skybox in show ground in the sky with clouds but that there is a job to do with the terrain.

Es can create a field as a hollow saucer? es whether can tell me how to proceed?

Thank you in advance for your reply

Sorry for my English I French