Kerr spacetime contains closed timelike curves inside the inner horizon. You can reach them from the outside universe, but you can't escape back out. Also, the

Kerr spacetime contains closed timelike curves inside the inner horizon. You can reach them from the outside universe, but you can't escape back out. Also, the region inside the inner horizon of Kerr spacetime is widely considered to be not physically reasonable, not just because of the closed timelike curves, but because the inner horizon itself is unstable--there is an infinite blueshift there which, it is believed, would cause it to be destroyed by the first tiny bit of incoming matter or radiation.