The Lost City of Faar

The Lost City of Faar is the second book in the Pendragon series by D. J. MacHale. 
the people live on immense, floating cities called habitats and grow food on the sea floor and in their habitats. Different habitats do different things such as producing food, making products, etc. It's an idyllic world until Saint Dane unleashes a poison that makes the crops grow large but fatal. To save the territory, Bobby battles pirates on the surface and killer sharks below, all in hopes of finding salvation on a legendary underwater city called Faar.
Plot Introduction.
Loor flumes to her home in Zadaa while Press and Bobby flume to Cloral. Most of the book takes place on the territory known as Cloral, a planet that is entirely covered by water. The inhabitants of Cloral live on giant floating barges, called habitats. The main habitat in the book is called "Grallion". Grallion is responsible for growing food for Cloral. A territory called Zadaa is also visited. Pendragon goes searching for a mythical city called Faar.
Plot.
After leaving for home after his first adventure on Denduron, Bobby finds out that his entire history and proof of existence had simply vanished, including his house. After seeing this, he decides to go to Cloral in order to stop Saint Dane, a demon traveler who wants the territories to go the wrong way, so he can recreate them in his own image. After Bobby, his uncle Press, and Loor, the traveler from Zadaa, arrive at the flume, Loor heads back to Zadaa and Bobby and uncle Press head to Cloral.
When they arrive, they find the clothes used by the people of Cloral, which were brought by acolytes, people on the territories who help the travelers. They also quickly realize that Cloral is an advanced society. They swim out of the cavern they arrived in, and they are attacked by sharks. Bobby is saved by Vo Spader, the traveler from Cloral. However, Spader does not know it yet. They board a jet-ski like vehicle called a skimmer, and head to Grallion, a giant barge called a habitat that serves as a city. Grallion is a farming habitat responsible for growing food on Cloral. When they arrive, Press informs Bobby that he has to tell Spader he is the new traveler of the territory soon.
The three of them stay there for a few weeks. One day, another habitat called Magorran, which is responsible for manufacturing, suddenly appears in the distance and is speeding full throttle towards Grallion. Despite auquaneer's and the barge officers efforts, the Barge does not slow down and Magorran crashes into Grallion. Bobby, Press, and Spader board Magorran, and find that everybody on Magorran is dead due to eating poisoned crops. Spader's father was also killed, although he gets half of a map that his father wanted him to have. Just as they were trying to figure out what happened on Magorran, Grallion was attacked by raiders (the Cloran equivalent of pirates) who bombarded Grallion with water cannons.
The raiders threatened to destroy Grallion with the cannons if Grallion does not pay a fee of ten cargo barges of food. The leader of the raiders said that apparently, all habitat's food, except for Grallion's, was poisoned too. Bobby recognizes the leader, Zy Roder, as Saint Dane in disguise. Spader comes up with a solution to save Grallion- diving over to the raider's ship, they disabled the engines which provided water for the cannons. However, halfway through their plan, they are discovered and chased by the raiders.
After the raiders chased them, Bobby decided to lead Spader to the flume. (Uncle Press was left on board.) Bobby then reaches the flume with Spader and activates it, taking both of them to Zadaa. The raiders were left behind. On Zadaa, after an encounter with a Rokador, they made it to the surface and went to a Batu (the name of the tribe) tournament to find Loor. Soon after, Bobby and Loor explained to Spader about how Saint Dane wanted to destroy all the territories in Halla. Sadly, Spader was only concerned with avenging his father by killing Saint Dane, whom they suspect poisoned the crops.
Bobby and Spader flume back to Cloral, where they find themselves heroes as Grallion defeated the raiders when their cannons were useless. Saint Dane (Zy Roder) escaped on a speedboat, though.
Bobby then advises Spader to go to Panger City, another habitat, to find his mother. When they get there, they discover that the only thing left is the other half of the map to the Lost City of Faar. It is then that Bobby, Press, and Spader are confronted by Rodar. He takes the map from them and looks at it, although they grab it back and escape to search for Faar.
Several hours later, they arrive right over the point where Faar is. Bobby, Press, and Spader sink down and enter the city, where they learn that the Faarians have invented something to cure the crops. They send out haulers (like a submarine) to start curing the crops. Because of this, Saint Dane knows where to strike missiles to keep the haulers inside. Press then tells Bobby and Spader to go get the people to evacuate the city, while he keeps trying to help.
Bobby and Spader warn the city and escape with them, and just afterwards the city collapses. Bobby decides to go with Spader back into the ruined city to find Press. After they find him, Bobby passes through the Council of Faar and find that the Old Man that he has talked to earlier in the book died desperately trying to reach a button. They decide to press it for him, and this causes Faar to rise back to the surface, so that now the haulers are able to get out. Saint Dane escapes, and they follow him to a second flume on Cloral, where they watch him escape to the territory of Veelox.