A confluence of circumstances that makes such a 'doorway' easy to open itself?
We conceive of time being something that is a flow from moment to moment, but I find very little thought on how the dimension of time is related to the three dimensions of space. Perhaps our attachment to horological devices, and the earth's near constant orbital rotation and period, essentially make this sort of illusion persistent.
But if this flow of time was not a linear progression, rather more like a stacking progression, sort of how you would stack 2 dimensional sheets of paper into a stack, or say a 3-dimensional layering like an onion, or a 4 dimensional undulation of a sphere known mathematically as a 3-sphere, you then technically wouldn't need much to disturb the arrangement of the flow, or the presentation of time at any moment recorded for all eternity on a planet. All you would need to do, is say eject enough of a harmonized current into some apparatus that generates some form of electromagnetic field, technically, or naturally, for such possibilities to maybe present themselves.
The guy at the Why Files presented some of his researched cryptid stories to be essentially sitting around the confines of underground caves, aquifers and water systems that have a grounded affinity, and interact with the electromagnetic system of the planet, producing fields of different variety as compared to those in more human based areas, in the video below:
It might be trifling, if it not were for another Youtuber, Bob Gymlan, writing up a viewer submitted story about a time-portal being opened, with the viewer and her relative being "transported" briefly but straight into the Carboniferous period, with massive dragonflies and such. That being the first narrative that Bob presented in this video below:
Something akin to this artistic rendition of the geologic period:
Of course, with everyone basically having a camera these days, we should have seen at least significant evidence of these time portals doing their thing ... ... ...
But this find by @Ricardo might be what you're looking for, if humans ever got down the technical details and technique of that sort of thing. (the html jumps around a bit, so you'll need to refresh the page once every embedded thing gets loaded in).
Some of these cryptids may be trans-dimensional "window fallers" like the Chupacabra or the big cats that have been seen in England in recent years. In other cases they may be like the Loch Ness Monster, which is trapped in a time warp:
As to the story about a time-portal being opened with the viewer and her relative being "transported" briefly into the Carboniferous period, this reminds me of stories about what are generally known as "time slips" that are far more common than people realise. If you want to learn more abut these, I would recommend British author Jenny Randles who has collected many accounts of such incidents and published them in works such as 'Time Storms' - see: Time Storms: The Amazing Evidence of Time Warps, Space Rifts and Time Travel: Amazon.co.uk: Randles, Jenny: 9780749921590: Books
As an introduction, you can find a sample of such tales at this link: Ten Amazing Slips In Time
Even my ex-wife experienced one in our old home town when she was passing a shop on the high street. She saw a woman locking a shop door but when she looked again, she could see no woman and the shop front had completely changed its design. This was somewhat reminiscent of story 9 in the 'Ten Amazing Slips in Time' involving the off-duty police officer visiting the bookshop in Liverpool. One reason why so many of these incidents seem to happen in England is that the C's have said that England is a "window" area, which was the reason why Stonehenge was built there. However, there are many such window areas scattered throughout the Earth including one in the Rennes-le-Chateau region in southern France:
As an introduction, you can find a sample of such tales at this link: Ten Amazing Slips In Time
Even my ex-wife experienced one in our old home town when she was passing a shop on the high street. She saw a woman locking a shop door but when she looked again, she could see no woman and the shop front had completely changed its design. This was somewhat reminiscent of story 9 in the 'Ten Amazing Slips in Time' involving the off-duty police officer visiting the bookshop in Liverpool. One reason why so many of these incidents seem to happen in England is that the C's have said that England is a "window" area, which was the reason why Stonehenge was built there. However, there are many such window areas scattered throughout the Earth including one in the Rennes-le-Chateau region in southern France:
Another such area would seem to be New York's Hudson Valley, a location connected to many strange accounts ranging from numerous sightings of UFOs to Bigfoot encounters and the existence of ancient portals.
For more see: 10 Encounters That Show the Hudson Valley Is a Fascinating Place
One ancient stone enclosure at a place named the Whang Hollow Portal is believed to be a gateway and may be as old as 3,500 years. It was most likely built by Celtic visitors or settlers, suggesting an ancient link between the Celts of Western Europe and America that long pre-dates Columbus and Leif Erikson, as was proposed by Professor Barry Fell in his controversial work America B.C.: Ancient Settlers in the New World first published in 1976 - see America B.C.: Ancient Settlers in the New World - Wikipedia. One reviewer of his book said that Fell's work opened "a portal into what seemed to be a lost world". Given what has been claimed about Whang Hollow Portal, this would seem to be a very apt description.
For more see: 10 Encounters That Show the Hudson Valley Is a Fascinating Place
One ancient stone enclosure at a place named the Whang Hollow Portal is believed to be a gateway and may be as old as 3,500 years. It was most likely built by Celtic visitors or settlers, suggesting an ancient link between the Celts of Western Europe and America that long pre-dates Columbus and Leif Erikson, as was proposed by Professor Barry Fell in his controversial work America B.C.: Ancient Settlers in the New World first published in 1976 - see America B.C.: Ancient Settlers in the New World - Wikipedia. One reviewer of his book said that Fell's work opened "a portal into what seemed to be a lost world". Given what has been claimed about Whang Hollow Portal, this would seem to be a very apt description.
