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Schwarzschild wormhole
The Schwarzschild metric admits negative square root as well as positive square root solutions for the geometry.
The complete Schwarzschild geometry consists of a black hole, a white hole, and two Universes connected at their horizons by a wormhole.
The negative square root solution inside the horizon represents a white hole. A white hole is a black hole running backwards in time. Just as black holes swallow things irretrievably, so also do white holes spit them out. White holes cannot exist, since they violate the second law of thermodynamics.
General Relativity is time symmetric. It does not know about the second law of thermodynamics, and it does not know about which way cause and effect go. But we do.
The negative square root solution outside the horizon represents another Universe. The wormhole joining the two separate Universes is known as the Einstein-Rosen bridge.