Escape from the Island of Aquarius
Escape from the Island of Aquarius is a 1986 Christian novel by Frank E. Peretti, and the first sequel to The Door in the Dragon's Throat in the Cooper Kids Adventure Series. In this installment, the Coopers travel to the island of Aquarius in the South Pacific to locate a missing missionary who was previously presumed dead.

Plot
In the South Pacific, a fishing boat finds a man lying dead on a makeshift raft, with a note bearing the name Adam MacKenzie.  Dr. Jake Cooper and his children Jay and Lila subsequently travel to the mysterious island of Aquarius to locate MacKenzie, a missionary who, until his note was found, had been presumed dead.

Following their arrival on the island, a Polynesian man who speaks little English but seems to recognize MacKenzie's name brings the Coopers to a village that seems more American than Polynesian, where the village's leader greets them and identifies himself as MacKenzie.  He explains that the man found on the raft, who was known as Tommy, was a victim of a deadly curse known as "Moro-Kunda"; the Coopers are skeptical of the idea of a curse.

A man appears by the edge of the forest surrounding the village and advises the Coopers to leave the island as soon as possible, and to take him with them.  Identifying himself as Amos Dulaney, a professor of geology, he explains that the island is not safe.  Before the Coopers can respond, however, MacKenzie and the village's armed guards confront Dulaney, and MacKenzie has Dulaney taken away, stating that he has been hit by Moro-Kunda.  That night, the Coopers decide to discreetly explore the forest, and find Dulaney's body, displaying the same symptoms that were found on Tommy.

The next morning, the Coopers leave the village, escorted by Candle, the Polynesian man who originally brought them to the village.  On the way back to their boat, however, Candle beckons the Coopers down another path.  The Coopers initially resist, but after the rope bridge connecting the main island to the harbor proves too precarious to cross, decide to see what Candle wanted to show them.  Following the path, they come across an abandoned and partially submerged village.

The Coopers then return to the main village and explore the forest again, this time finding a pit containing what appear to be animal and human bones, and suspect it to be used for sacrifices.  However, they are then confronted by MacKenzie and his men, and MacKenzie claims they have now been hit by Moro-Kunda and has them locked in the island's "confinement hut".  The Coopers eventually escape after Dr. Cooper lures the guard in and knocks him out.  He shows Jay and Lila a dead fly on the ground, which he had killed after it was inserted through a peephole in the hut's wall, and identifies it as a highly venomous species and the cause of the symptoms found on Tommy and Dulaney.

After locking the guard in the hut, the Coopers find the inhabitants of the village partaking in firewalking and kneeling in front of MacKenzie after crossing the embers.  After inferring that the villagers appear to consider MacKenzie a god, the Coopers search MacKenzie's empty cottage and find a photo from which Dr. Cooper draws an unspecified conclusion, and a cage of flies, confirming that MacKenzie planted flies on Tommy and Dulaney as he had on the Coopers.  MacKenzie and his men surround the hut, but Dr. Cooper holds the cage up and forces MacKenzie and his men to retrieve their possessions and let them return to their boat.  Once Dr. Cooper and Jay are on the bridge, however, MacKenzie has his men seize Lila and destroy the bridge, sending them into the whirlpool below.

Dr. Cooper and Jay soon feel themselves pulled onto dry land in a large cavern by a man Dr. Cooper recognizes from the photo as Adam MacKenzie, who identifies the village's leader as Stuart Kelno.  Adam explains that he had initially been successful in his mission to Aquarius, establishing a Christian community in the now-abandoned village, until Kelno and his followers arrived.  The Christians fled the island, while Adam was dropped into the whirlpool like Dr. Cooper and Jay, and has been living in the now-empty inside of the volcano that formed the island ever since.  When Dr. Cooper recalls hearing the name Kelno from Dulaney, Adam reveals that Dulaney had been one of Kelno's main supporters.  Adam has built a large boat with the materials that had been meant to be used to build a church, which he calls "Adam's Ark".

Adam also reveals that the island is sinking, as its foundation has become unstable due to erosion.  When Jay comments on the draft coming from the cavern's entrance, Dr. Cooper realizes that it is actually an air current, and that there is another way out of the cavern.  The three find a tunnel leading up from the cavern and start up it.

Meanwhile, Kelno has Lila prepared to be put into the pit as a sacrifice.  After he properly introduces himself to Lila, who had realized that he was not Adam MacKenzie, Lila is lowered into the pit, where she comes face to face with a large serpent.  After the serpent bites off the ceremonial gown Lila had been dressed in, she is pulled out of the pit by Candle, who takes her to the abandoned village and reveals that he is a Christian and a friend of Adam.  He tells Lila about the cavern and that Adam, Dr. Cooper, and Jay are alive.  He then takes Lila out on his dugout canoe to show her the cavern entrance, but they find that it is now submerged.

Adam, Dr. Cooper, and Jay find that the tunnel crosses a deep rift caused by the island beginning to split apart, but they are able to cross it.  They then catch sight of the serpent before emerging in the pit, where they find the remains of the gown Lila had been wearing.  After climbing out of the pit, they return to the main village, where Dr. Cooper confronts Kelno at gunpoint.  After Kelno calls him out for his un-Christian behavior, Dr. Cooper backs down, and offers to save Kelno from the sinking island.  They go outside, where Adam addresses the villagers and invites them to come with him on his boat.  Kelno attempts to dissuade the villagers, but most of them go with Adam after the rift reaches the surface and begins to destroy the village.  Adam leads the villagers to the pit, and he, Dr. Cooper, and Jay are able to get everyone back over the rift.  When they reach the cavern and board Adam's boat, however, they find the entrance underwater.

Lila asks Candle to row back to the abandoned village, where she first asks him to secure the canoe in order to preoccupy him.  She then takes the box of explosives the Coopers had brought with them, which Candle had previously moved to the supply hut from the Coopers' boat, along with an oxygen tank, and jumps into the water near the whirlpool.  She plants explosives along the outside of the cavern wall and sets them to detonate after a delay, but is soon pinned by a piece of rock that falls from above her.  With her oxygen nearly depleted, Lila quickly detonates an explosive on the rock, freeing herself, and swims up to the surface.

Kelno sends his men after Adam's group, but when they find themselves trapped in the cavern, they drop their guns and join Adam's group.  As the water rises in the cavern and the boat approaches the cavern's ceiling, Adam prays for everyone as they believe they are doomed, but then the explosives planted by Lila detonate, destroying the wall, and the boat rises up to the surface.

As the island becomes flooded, Candle, who had been looking for Lila, climbs to the top of a tree, while Kelno decides to try to escape through the pit.  However, the serpent eats Kelno seconds before the pit is flooded.

The company on Adam's Ark finds Lila and brings her aboard, followed by Candle.  As the island disappears fully and the water begins to calm, the company on Adam's Ark finds Candle's still-loaded canoe, and, with Adam preaching to the company, departs for the nearest island.