The "Sentinel" arrives in New York. By this time, the cavern is completely flooded and Ned and Joe have been forced to put on their equipment, which gives them enough air for no more than two hours. Launching "Thunderbird 4", Gordon locates the entrance to the subterranean river and proceeds towards Ned and Joe's position. At ground level, Scott learns that the subsidence has expanded and is now threatening to bring down the Fulmer Finance Building. Knowing that Ned and Joe will not survive the impact of a second skyscraper collapse, Scott urges them to swim along the river to meet "Thunderbird 4". Despite their air supply being virtually exhausted, the men are rescued and removed from the area just as the Fulmer Finance Building collapses, triggering an underwater shockwave.