In the fictional universe of the Batman comic books, "Lazarus Pits" are pits filled with a mysterious liquid that can heal illness, cure injury, prevent aging, and even bring the dead back to life (hence their being named after Lazarus, the man Jesus Christ raises from the dead in the New Testament).
The Lazarus Pits, which occur naturally at the intersection of ley lines, were the secret of supervillain Ra's Al Ghul's immortality, as every time he suffered an injury or was killed, he would have himself placed in a Lazarus Pit by his followers. An individual can only use a certain Lazarus Pit once, so much of Al Ghul's life was spent seeking out new pits to keep extending his lifespan.
In addition to its healing properties, being placed in a Lazarus Pit has two temporary side effects - for a short time, the user has incredible, inhuman strength, and is also temporarily insane, often attempting to kill those around them. Placing a fully healthy person in a Lazarus Pit will generally kill them.
The properties of the Lazarus Pits were first discovered by Ra's Al Ghul over 600 years ago, when he was just an ordinary member of a tribe of nomads in the Arabian dessert, and used the pit to save the life of a dying prince. But during the prince's temporary insanity after using the pit, he strangled Al Ghul's wife Sora and tortured Al Ghul before leaving him to die in the desert. This prompted Al Ghul to use the power of the Lazarus Pit to heal himself and get revenge on the prince.
Eventually, Al Ghul would go on to establish an international eco-terrorist organization as part of his effort to restore the Earth to environmental balance. Hundreds of years later, his nemesis, the Batman, would destroy nearly all of the natural Lazarus Pits in the world in an effort to cut off the source of Al Ghul's power. Batman only later learned that the Lazarus Pits were crucial to allowing the natural cycle of life to continue, but fortunately by that point, a method of artificially producing Lazarus Pits had been developed.