How deep is the sea? Pipkin the inquisitive penguin wants to know just that. With the help of a friendly seal, a big blue whale and a salty sea dog in a yellow submarine, Pipkin learns that the sea is very deep indeed! A delightful sequel to Pipkin's adventures in "How big is a million?" and "How High is the Sky?" How deep is the sea? Pipkin the inquisitive penguin wants to know just that. With the help of a friendly seal, a big blue whale and a salty sea dog in a yellow submarine, Pipkin learns that the sea is very deep indeed! A delightful sequel to Pipkin's adventures in "How big is a million?" and "How High is the Sky?"