"It's Started"

In the final scene of "Amazing World of Gumball", Rob wakes up alone in Elmore High School. A second later, the ground underneath him cracks and breaks apart, revealing the Void beneath. Rob realizes the disaster has begun, just before falling in. The final image is a frozen shot of him falling as a dark rendition of the show's theme song is heard.

Example of:
Bolivian Army Cliffhanger