Complex sentence: Alfie wonders how the ship got into the cavern, Jack thinks that it maybe like a model ship in a bottle but Donald and Douglas believe that their must have been away to sail into the cavern.
Split into simpler sentences: Alfie wonders how the ship got into the cavern, Jack thinks that it maybe like a model ship in a bottle. But Donald and Douglas believe that their must have been away to sail into the cavern.