The Prestige

In early 1900s London, magician Robert Angier performs his trick, 'The Real Transported Man,' to a sold out theater. Rival magician Alfred Borden, in disguise, sneaks under the stage. At the trick's culmination, Angier drops through a trap door and into a waiting water tank, where Borden watches him drown.
At Borden's murder trial, Angier's ingÃ©nieur, John Cutter testifies how Borden pushed the water tank under the trap door to catch and kill Angier during his New Transported Man trick. In prison, Borden is visited by an agent of Lord Caldlow, who offers to care for Borden's daughter Jess in exchange for all of Borden's tricks. As a show of good faith, Caldlow gives Borden a copy of Angier's diary, which Borden reads.
In flashback, Angier and Borden work as shills for Milton the Magician, alongside ingÃ©nieur John Cutter and assistant Julia, who is also Angier's wife. Milton's famous trick is the water tank trick, where Julia is bound in ropes and dropped in a water tank, from which she frees herself and escapes. Borden, with Julia's consent, ties her hands with a more secure, yet difficult knot. Julia fails to slip the knot and drowns, infuriating Angier and fueling his deep-seated grudge against Borden.
Borden launches a solo magic career and hires the silent, mysterious Fallon as his ingÃ©nieur, Borden courts and eventually marries Sarah, who gives birth to their daughter, Jess. At his first show, Borden's bullet catch trick is sabotaged by Angier, and Borden loses two fingers. Angier launches his own magic career, hiring Olivia Wenscombe as his assistant and Cutter as his ingÃ©nieur. During the finale of Angier's show, a disguised Borden sabotages Angier's bird cage act, which maims an audience member and ruins Angier's reputation.
Angier discovers, and subsequently steals Borden's fantastic trick "The Transported Man," where Borden instantly travels between two wardrobes on opposite ends of a stage. Cutter and Olivia groom Root, an out-of-work actor, into a "double" for Angier, to mimic his appearance and mannerisms. 'The New Transported Man' is a huge success, but Angier is displeased, as he ends the trick hidden under the stage while Root basks in the applause. Obsessed with Borden's secret, Angier orders Olivia to spy on him. Instead, she revamps Borden's act, making it more successful. Borden subsequently sabotages Angier's show, humiliating him and leaving him with a permanent limp. Angier confronts Olivia, who confesses to loving Borden, before giving Angier a copy of Borden's diary, its contents scrambled by a coded cipher. Angier and Cutter kidnap Fallon and extort from Borden his codeword "Tesla" which Borden claims is also his secret. As Angier leaves for America to find Tesla, Borden begins an affair with Olivia. Sarah becomes increasingly disturbed with Borden's fickle and contradictory nature and, succumbing to depression, commits suicide.
In America, Angier meets scientist Nikola Tesla and asks him to build him the machine Tesla built for Borden. While waiting for Tesla to finish, Angier deciphers Borden's diary, which he discovers to be fraudulent: Borden confesses that Tesla has nothing to do with Borden's trick and that Borden ordered Olivia to give Angier the diary to send him away. Angier confronts Tesla, who admits to never building any device for Borden, but has successfully built a replicating machine for Angier, capable of reproducing animate and inanimate objects a distance from the machine. As Tesla departs, he advises Angier to destroy the machine.
Returning to London, Angier debuts 'The Real Transported Man' where he vanishes within the machine's electrical field and appears atop the balcony at the back of the hall. Baffled by the trick's success, Borden sneaks backstage to find Angier's secret, only to witness Angier drop through a trap door and plunge into a waiting water tank, where he drowns. Cutter stumbles upon the scene, and Borden is arrested.
In the present, Borden is found guilty and sentenced to death. He agrees to Lord Caldlow's terms, and surrenders all his secrets. When Caldlow visits, Borden recognizes him as Angier. Baffled by Angier's return, Borden begs for his life, but Angier ignores Borden's pleas. Borden is hanged and dies, his last word: "Abracadabra." Cutter learns that Caldlow has bought Angier's machine. When he visits him to plead for its destruction, he discovers that Caldlow is Angier, and faked his death to kill Borden in revenge for Julia. Disgusted with Angier, Cutter agrees to help Angier dispose of his machine in a private theater. As the two work, Cutter discovers rows of water tanks containing rotting duplicates of Angier: Every show, Angier used the machine to create clones of himself atop the balcony, while unbeknownst to anyone, the original Angier would fall through the trap door to be drowned in the tanks under the stage. The night Borden ventured backstage, Angier's clone presumably disappeared into the crowd, making it appear Borden had killed him under the stage.
His task finished, Cutter leaves the theater, allowing a man to enter as he departs. The man shoots Angier and reveals himself as Borden. Angier realizes that "Alfred Borden" is actually an identity assumed by twin brothers who took turns being Borden and Fallon, each living half of Alfred's life to prepare for their "Transported Man" trick, right down to each having separate spouses, Sarah and Olivia. Borden berates Angier for going too far in his quest for dominance. Angier supports his actions by explaining that all he ever wanted was to amaze an audience. If he could have fooled them even for a second, then his work would have been complete. Angier dies and the living Borden twin leaves to pick up Jess at Cutter's workshop, exchanging a silent goodbye and leaving. Where on the ending note shows the Clone of Angier in the glass chamber, near the dead body. Cutter narrates that, while we try to find the secret of a magic trick, we will never find because we don't really want to. Instead, we want to be fooled.

Question: What did Borden push under the trap door?
Answer: Water tank; Water tank