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In page Wormhole:

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For a simplified notion of a wormhole, space can be visualized as a two-dimensional surface. In this case, a wormhole would appear as a hole in that surface, lead into a 3D tube (the inside surface of a cylinder), then re-emerge at another location on the 2D surface with a hole similar to the entrance. An actual wormhole would be analogous to this, but with the spatial dimensions raised by one. For example, instead of circular holes on a 2-Dimensional plane, the entry and exit points could be visualized as spherical holes in 3D space leading into a four-dimensional "tube" similar to a spherinder.[citation needed]