- "What a maiden voyage this has turned out to be. Even the weather's deteriorating."
— Ocean Pioneer I Commander
Lost in a mysterious fog while at sea, the tanker Ocean Pioneer suddenly explodes - for no apparent reason. It's only after a sistership, Ocean Pioneer II, has been built, and sent on an identical mission, that Brains discovers that Liquid Alsterene - the tankers' volatile cargo - becomes highly-explosive, in the presence of a certain variety of sea algae...
- Main article: Danger At Ocean Deep/Storyline
- Main article: Danger At Ocean Deep/Transcript
International Rescue Equipment Used
Non-International Rescue Equipment Used
- This is the only episode in which John goes on a rescue.
- At the end of the episode, John tells Jeff that he's been on "a dozen" rescue missions. Scott claims that he's been on "all of them." (This may be seen as an attempt at some brotherly "one-upmanship" on Scott's part, especially in front of their father; notwithstanding that in The Perils of Penelope, he'd just returned home from taking leave, and then learned that a rescue was already underway without him).
- Lord Worden was previously seen as Sir Jeremy Hodge in The Perils of Penelope, while the mayor (on Penelope's right, at the launching of Ocean Pioneer II) was previously seen as Professor Holden in The Mighty Atom.
- Also visible in the crowd are Lord Silton, Tidman (from The Man From MI.5) and Bob Williams (from Cry Wolf).
- Much of the music from this episode is recycled from the Stingray episode The Big Gun, The Ghost Ship
- The music played during the launch of Ocean Pioneer II is recycled from the Fireball XL5 episode Flying Zodiac, and from the Stingray episode Stand by for Action. (It was also re-used later in the Captain Scarlet episode The Launching, during the launching of the ship President Roberts.)
- Ironically, Virgil accidentally contributes to Ocean Pioneer II's destruction when one of Thunderbird 2's thrusters collides with the sea tanker's antenna, setting it alight.
- Alan Perry was an uncredited camera operator on this episode.
- This is the only aquatic-related mission to not utilize Thunderbird 4.
- The filming miniature of Ocean Pioner and the deck model were seen in the Captain Scarlet and the Mysterons episode The Launching.
- Main article: Danger At Ocean Deep/Goofs
- French: Danger sur les mers
- German: Gefahr aus dem Meer
- Spanish: Alerta en el Fondo del Océano; Peligro en alta mar (DVD)
- Italian: Pericolo in alto mare
- Dutch: De kokende zee (TV); Gevaar in de diepe zee (VHS, DVD)
- Portuguese: Mistério em Alto Mar
- Japanese: オーシャンパイオニア号の危機