You go in dead. You come out insane. 
History
A Lazarus Pit is a natural phenomenon that possesses restorative properties that instantly heal injuries and even grant immortality. The origins of the Lazarus Pit were accidentally discovered by Ra's al Ghul while digging a pit. There he discovered the restorative chemical pools while searching for a cure for a dying prince. Although the Lazarus Pit saved the prince's life, it also drove him mad and caused him to kill Ra's al Ghul's wife, Sora. This incident led to a series of cruel circumstances for Ra's al Ghul, eventually turning him into the feared "Demon's Head". For several centuries, the Lazarus Pits were used by Ra's al Ghul to prolong his life.[citation needed]
By the time Ra's met Batman, his life force was almost depleted and he eventually died. Ra's and his men had traveled to Switzerland in order to use one of Ra's Lazarus Pits in his secret hideout on the snowy mountains. Batman tried to capture Ra's and bring him to justice, but Talia convinced Batman that Ra's was dead and after Batman was gone, she activated the machine that was constructed over the Lazarus Pit with a mechanism that placed Ra's in the pool for a few minutes before bringing him back to the surface.[1]
The Lazarus Pits have been later used by multiple people across the globe, including Ra's al Ghul's daughter, Nyssa, Black Canary[2], the Riddler,[3] Lady Shiva, Cassandra Cain and Kate Kane.
Ra's's daughter, Talia used the Lazarus Pit to heal Jason Todd's mind and body after his unexpected resurrection.[4]
The fanatical terrorist known as Kobra also had his own hidden network of Lazarus Pits after having analyzed and duplicated the chemical compositions of the Pits. Furthermore, Kobra's analysis of the Lazarus Pits' chemicals allowed him to control those he resurrected as mindless slaves.
Eventually all of the Lazarus Pits were apparently destroyed by Bane and Batman, except for one. Because each Lazarus Pit can reportedly only be used one time,[citation needed] Ra's had to accept the fact that eventually he would die (a notion that is subject to debate, as there is evidence of him existing in the 31st century). His daughter Nyssa, however, had discovered a way to make the Lazarus Pits last indefinitely, but rather than let her father use it, she stabbed him to death. Although supposedly only one pit remains, its location has appeared vague — at one point it appears to be in an island compound Ra's assaulted. Later it appeared to be in a completely different location, and another "last" pit was constructed inside the Batcave. This may in fact indicate that there is more than one pit remaining, or that Nyssa was able to create additional pits.
Powers
The substance that fills each pit is a chemical blend of unknown composition, seemingly originating from somewhere within the Earth's crust. For an unknown reason, it bubbles to the surface only at certain key points around the globe, typically at the junction of ley lines. The chemicals can rejuvenate the sick and the dying (and even the technically deceased as long as they have not yet truly died), but will typically kill a healthy user. In addition, it appears to act as a youth restorative sometimes, decreasing the age of a user depending how long they were in the pit, and in many cases, resurrect the dead.
For maximum efficiency however, someone must be placed in the Lazarus Pit within a relatively short period of time following their death. Although older bodies may be resurrected, there is no guarantee that they will return in perfect condition. On one occasion, Ra's exhumed the bodies of Thomas and Martha Wayne, threatening to immerse them in the pit, thus transforming them into undead zombies.[5]
When used to heal or to bring someone back to life, that person is also rendered temporarily insane — although in one instance when it was used for the Joker, the reverse effect occurred, i.e. it temporarily rendered him sane).[citation needed]
Trivia
- The Lazarus Pit is named after Lazarus, the biblical figure whom Christ resurrected from the dead.[6]
- The Church of Blood possesses a device that functions similarly to that of a Lazarus Pit. Raven once used one of these pits to resurrect her former college classmate, Jericho.[7]