Portal haunting is widely debated as little is known. It would seem that the idea of doorways exist to allow entrance into this plane from another is merely conjecture at this point in time. Some ghost hunters believe that portals exist between the physical and spirit worlds. It is theorized that portals would then act as a door for ghosts and spirits to cross over from the other worlds unto the physical realm. And if portals exist, then it is speculated that haunting, paranormal activity, would increase near these doorways into our universe. If true, portal haunting might be indicators of invisible gateways between the earth and other dimensions - even other worlds! Yet, no one knows for sure if portals actually exist, and if so, what they may be.
It is quite common for paranormal investigators to believe that strange entities (beings that appear other-worldly and do not seem human) captured in ghost photography are indicators that specific areas of our world might have higher parnormal activity, and thus, are investigated as potential portals. It is believed by some ghost investigators that the portal haunting has a high occurrence around graveyards or near underground waterways (perhaps due to the increased energy being caused by flowing water over rock). Some have fostered the notion that graveyards may have been built, unknowingly, yet guided by the subconscious or higher powers that be, in areas that portals exist. But again, there is no proof of any of this, only theory.
With all this said, we simply do not know if portals exist at all. It is interesting to note that most near death experiencers recall going through a tunnel of light during their crossing over from this world to the next. Is a tunnel of light a portal and can it work in the opposite direction, as a two-way door for higher spirits? Is there something similar to allow ghosts, or disembodied persons, to come back to earth and visit? And might there be other tunnels, or portals, that may explain some of the strange type of haunting people have witnessed?
Finally, we should note that in ghost photographs, many ghost hunters, especially novices, strongly believe in the idea of "vortexes" as evidence of these portals. A vortex usually appears as a streak of white ectoplasm mist, and is typically confused with photos of accidentally-photographed camera straps captured hanging in front of the lens. For more on this, visit Vortex Ghost Pictures.