Doraemon Movie (1987)

Nobita and Doraemon were finding out the truth about a real dinosaur. Nobita saw something under the ocean when they come across a cave that leads to an underground world full of dino-people. Nobita and his friends are amazed by their new discovery but sadly their memories must be wiped and must return to earth.

Original title: Doraemon: Nobita and the Knights on Dinosaurs (1987)