Once Thomas had to deliver beach supplies to the Seaside Halt and Diesel warned him about "the curse of the cliffs" which meant that he might get lost and never be seen again. However, Thomas plucked up courage and discovered that nothing bad happened and that Diesel was only trying to scare him.
When they were first introduced in CGI, there was a small, rocky island next to the coastal cliffs. However, since the move to Arc Productions from the seventeenth season, the island has disappeared.