The tension builds up over the question of whether or not the main character Kernans will depart as the rest of the soldiers and scientists head for the cooler climate of the North. Beatrice, who Kernans seems to be semi-enchanted by, it is clear is not leaving so that raises the prospect that there might be others prepared to stay.
One of them emerges early with one of the soldiers Hardman running off to follow his desire to head for the South. That leaves Kernans, who starts having dreams that are the start of some primeval urge to return to the swamps, waiting to weigh up his own mind.
The decision to stay, which you always suspected, comes quickly and Kernans is not alone with fellow colleague Bodkin helping him scuttles the scientific research station and retreat to different corners of the lagoon. Bodkin cannot leave because of his memories of London under the water and Kernans cannot leave because like Hardman, like all the dreamers, he is being called back to the South.