The mystery of the missing sea is one that has personally intrigued me recently. Through all of the Welsh history I have covered on this channel, I have never encountered a phenomenon as mysterious as what I've dubbed as the "islands that aren't islands".
Throughout a small valley in Wales, near the towns of Porthmadog and Tremadog, there are 10 hills that aren't called hills at all, but "islands", despite being completely away from the ocean, a phenomenon that is completely unique throughout the history of Wales.
Wales' history is full of mysteries, and these islands that aren't islands, surrounded by a missing medieval sea, is another one of them.

Chapters:
0:00 - Islands that Aren't Islands
0:45 - Ynys, Ynys, Ynys
2:52 - Traeth Mawr
5:10 - How to Remove 13 Miles of Coastline (100% Effective!)
7:35 - William Madocks

Sources (turn on captions):
Hislop, M. (2024). A Guide to the Medieval Castles of England. Pen & Sword History,
[1] pp. 175,
[2] 176.

Llyfrgell Genedlaethol Cymru / National Library of Wales. Cambriae Typus.
[3] https://www.library.wales/discover-le....

Rhind, P.M. and Jones, A. (1994). Brackish saltmarsh communities in the Glaslyn marsh trust reserve. Field Studies, 8,
[4] p. 373.

Robinson, G. and King, T.J. (2019). At Home on the Waves: Human Habitation of the Sea from the Mesolithic to Today. Berghahn Books,
[5] pp. 145-146,
[6] 146,
[7] 146-148,
[8] 147.

Thomas, D., (1959). MADOCKS, WILLIAM ALEXANDER (1773 - 1828), industrialist and philanthropist. Dictionary of Welsh Biography.
[9] https://biography.wales/article/s-MAD...

Tweet by @KeithAPettit:
[10]   / 1742951095784071278  

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Maps:
© OpenStreetMap contributors, licensed under CC BY-SA: https://www.openstreetmap.org/copyright
https://www.floodmap.net/

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Music courtesy of the YouTube Audio Library:
Rain Cloud Music - Emily A. Sprague
Sunrise Over Big Data Country - Dan Bodan
A Slow Dream - Emily A. Sprague
Stuck In The Air - The Tower of Light
Ammil - The Tides

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Images of, and from:
Afon Glaslyn and Traeth Mawr by Bill Boaden, CC BY-SA 2.0 https://creativecommons.org/licenses/..., via Wikimedia Commons
A sea of grass, south-west of Tremadog by Christine Johnstone, CC BY-SA 2.0, via Wikimedia Commons
Cnicht From the Cob, Porthmadog, Gwynedd by Peter Trimming, CC BY-SA 2.0, via Wikimedia Commons
"Island" near Farmyard by Dewi, CC BY-SA 2.0, via Wikimedia Commons
Y Cob, Porthmadog o'r awyr, Gwynedd, Cymru (Wales) by Llywelyn2000, CC BY-SA 4.0 https://creativecommons.org/licenses/..., via Wikimedia Commons

all other images are public domain, via the Yale Center for British Art, the British Library, and the National Library of Wales

#wales