"Operation Depthprobe" is a comic-strip story originally published in TV Century 21 issues 105 to 109, 21st January to 19th February 2067 (i.e. 1967).
Thunderbirds 1 and 2 save an airliner from crashing into the Nevada Desert. Scott and Virgil are bemused by the dismissive attitude of its passengers, who walk into the desert and then vanish beneath its surface. They are part of the top secret Operation Depthprobe...
Flight 287 from Tokyo is heading towards a top-secret location in the Nevada Desert when Pilot Joe Rand discovers that the controls are jammed with increasing speed; even advice from his base doesn't help as they'll be out of fuel within sixteen minutes. Luckily for Flight 287, John Tracy aboard Thunderbird 5 hears the distress call, and Thunderbirds 1 and 2 are quickly launched. Positioning their aircraft on each side of Flight 287, Scott and Virgil bring the plane out of the nose-dive and safely to landing.
However, not long after touchdown, a World Security officer orders International Rescue to depart at once, leaving Scott and Virgil feeling baffled; the passengers are in the middle of the Nevada Desert. Scott reports the situation to his father, Jeff Tracy who reminds his son that even they have to remain secretive.
Underneath the Nevada Desert lies one of the highly guarded secrets of 2067 - Operation Depthprobe, a fuel-carrying rocket containing a very unstable fuel known as pentatholeum, which must be kept refrigerated. However, whilst the final safety checks are carried out prior to launch, one man by the name of Spencer is removing a nut causing liquid to drip onto the cables; what his colleagues don't know about Spencer that he's actually a Bereznik agent sent out to destroy Western America.
So, at 14:00 hours the following day, astronauts Philips and Morris enter the nose cone, and two hours later, the countdown commences. However, at the point of lift-off, the rocket motors fire, but Operation Depthprobe hasn't taken off; worse still, the temperature's rising and the fuel-store refrigeration won't be able to take the pressure. If that blows, so too would their base plus Nevada itself! Two men bravely attempt to shut down the rocket motors manually, but the gantry to the nose cone collapses; luckily, both men escape unharmed.
Meanwhile, aboard Thunderbird 5, John is listening to the whole situation, and Jeff tells him to call them at once. John tells the World Security Officer at Operation Depthprobe that their secret won't be exposed, as International Rescue also must remain secretive, which he agrees to. Soon, Thunderbirds 1 and 2 are on their way, and upon arrival, Scott learns that they have twenty minutes before the fuel-store refrigeration gives out. Upon landing, Thunderbird 2 unloads the firefighting vehicles; first task is to damp down the fire. However, just as Scott and Virgil (with their jetpacks) attempt to remove the electrical circuit cover, Brains warns them to get out of the way - the whole rocket is toppling over, and they get clear just in time. The Depthprobe rocket is now leaning against the launch bay wall, and the temperature gauges are rapidly rising to dangerous levels.
On Brains' orders, Scott and Virgil get the firefighting vehicles in place to cool down the rocket, and then, with assistance from the security guards, they place fuel lines from the rocket into the pod of Thunderbird 2. However, just as the fuel pumping commences, the firefighting vehicles' water supply runs dry. With time running out, International Rescue must get the crew out of the nose cone at once.
Scott and Brains, attached to the side of the nose cone, remove the panels and then, Virgil aboard Thunderbird 2 lowers the harnesses and lift the two man crew up to the surface, closely followed by Scott and Brains on their jetpacks. Eventually, there's a massive explosion; but luckily, the base is undamaged.
However, Brains can't figure out the cause of the disaster so, whilst Virgil commences the fuel pumping from Thunderbird 2 back into the rocket's storage tanks, he checks the rocket's plans and discovers that the cables were corroded away. The World Security Officer realises that Spencer, who was responsible for that area, has disappeared, and Brains concludes that this was no accident - it was sabotage. So, the aircraft of Base Security are launched, and soon, they track down Spencer, ready to arrest him.
Then, with their mission completed, International Rescue set off for home whilst Operation Depthrobe recommences.