Osireion
The Osirion or Osireion is an ancient megalithic structure located at Abydos, to the rear of the Temple of Seti I. Its original purpose is unknown.
It is an integral part of Seti I's temple complex and is possibly built to resemble an 18th Dynasty Valley of the Kings tomb. The site contains a stone-paved island in the centre, chambers in both wings and, around the island, a water basin of yet undetermined but no less than 10.4 m depth which was probably used as a well. Access was via a 69 m stone lined passage. In the whole of Egypt, there is no architectural equivalent to the Osireion with its massive blocks, numerous trilithons, extraordinarily narrow and regular joints as well as a lack of round pillars except for the Valley Temple in Giza.
The Osireion was discovered by archaeologists Flinders Petrie and Margaret Murray, who were excavating the site in 1902–03. The Osirion was originally built at a considerably lower level than the foundations of the temple of Seti, who ruled from 1294 to 1279 BC. It is 17 meters (55 feet) below grade. While, with regard to the depth of the building, its untypical architectural style and its mythological context, there is disagreement as to its true age, in his 1998 dissertation on Seti I, Peter J. Brand, mainly emphasising on the numerous cartouches of Seti I found inscribed at the site, says it "can be dated confidently to Seti's reign".The Egypt Exploration Society (EES) which carried out the excavations of the site had come to this same conclusion, as detailed in its 1933 two volume excavation report. But nevertheless doubts have persisted. “In 2012, the SCA in cooperation with James S. Westerman, a Chicago engineer, attempted to drain a section of the canal surrounding the island in the Central Hall to inspect the foundation for the platform that supports the island.  If successful, this could have provided the answer as to whether: it is built directly on the “gebel moiya” layer as Frankfort had proposed; on a platform of Nineteenth Dynasty sandstone blocks; or on the kernel of a more, or possibly much more, ancient structure as Westerman suspects.  Unfortunately, they were only able to lower the water level by about 4 meters (13 feet) before the upward water pressure overwhelmed the pumps.” For more on this attempt see his website and the ongoing exploration by the ISIDA PROJECT listed below.
Strabo visited the Osireion in the first century BC and gave a description of the site as it appeared in his time:

Above this city [Ptolemaïs] lies Abydus, where is the Memnonium, a royal building, which is a remarkable structure built of solid stone, and of the same workmanship as that which I ascribed to the Labyrinth, though not multiplex; and also a fountain which lies at a great depth, so that one descends to it down vaulted galleries made of monoliths of surprising size and workmanship.
The subterranean water basin at the Osireion was consequently named "Strabo's Well" by modern excavators.
Strabo then theorises that the Osireion might have been constructed by Ismandes, or Mandes (Amenemhet III, 12th dynasty, who reigned in the 19th and 18th centuries BC), the same purported builder as with the Labyrinth at Hawara:

But if, as they say, Memnon is called Ismandes by the Egyptians, the Labyrinth might be a Memnonium, and the work of the same person who constructed those at Abydos and at Thebes; for in those places, it is said, are some Memnonia.
Strabo's uncertainty about the origins of the structure highly suggests that these were already unknown by the time of his visit. Fortunately, the EES excavations uncovered its identity. “Menmaatre [Seti I] is Benefical to Osiris” was the name given to the subterranean structure. Graffiti by Egyptian priests of Dynasties XXI-XXII found in the long Entrance Passage, the vaulted galleries mentioned by Strabo, hint at its function. “The hidden/secret place of the Underworld/Duat of Menmaatre [Seti I]” and more specifically and revealing as the “Birth House of Isis”.

See also
List of megalithic sites

References
External links
The Osirion The original notes by archeologist Sir William Flinders-Petrie on the discovery of the Osirion site, published in 1903.
Archeological exploration in Abydos, Egypt. Exploration work, conducted by James S. Westerman scince 1986 to 2012.
ISIDA Project  Abydos expedition report with historical background.
Study of the Osireion at Abydos. Code book and source document for the birth house of Isis.  Charles Henry Herzer Jr. Published in 2023.

Bibliography
Caulfeild, A.  The Temple of the Kings at Abydos (1902, reprint 1989). https://archive.org/details/templekingsatab00petrgoog/mode/2up
Frankfort, H., De Buck, A. & Gunn, B. The Cenotaph of Seti I at Abydos, 2 vol. Memoir of the Egypt Exploration Society, Egypt Exploration Society. London, 1933. listing:  (39th Memoir). https://archive.org/details/MEEF39.1/ and https://archive.org/details/MEEF39.2/ 
Hamilton, Keith. The Osireion: A Layman's Guide, 2018, Web, https://www.researchgate.net/publication/328225133_The_Osireion_A_Layman%27s_Guide.
Herzer, Charles H. Study of the Osireion at Abydos  Code Book and Source Document for the Birth House of Isis, 2023.  Text and Plate volumes: https://isida-project.ucoz.com/ Synopsis: https://www.academia.edu/92557861/
Hornung, Erik, trans. from German by David Lorton. The Ancient Egyptian Books of the Afterlife. Cornell University Press, Ithaca, New York, 1999.
ISIDA PROJECT.  https://isida-project.org  https://isida-project.org/egypt_april_2018/osirion_en.htm
Murray, Margaret A. The Osirion at Abydos, British School of Egyptian Archeology n°1, London, 1904.
Murray, Margaret Alice., J. G. Milne, and W. E. Crum. The Osireion at Abydos. London: Histories & Mysteries of Man, 1989. Print.
Naville, Edouard. "Abydos". The Journal of Egyptian Archaeology, vol. 1, no. 1, 1914, pp. 2–8. JSTOR, www.jstor.org/stable/3853664.
O'Connor, David. Abydos: Egypt's First Pharaohs and the Cult of Osiris, Thames & Hudson, London, 2009 and 2011.
Petrie, Flinders. The Osirion at Abydos
Pinch, Geraldine. Egyptian Mythology: A Guide to the Gods, Goddesses, and Traditions of Ancient Egypt. Oxford University Press, 2002.
Roberson, Joshua, A. “An Enigmatic Wall from the Cenotaph of Seti I at Abydos” JARCE 43 (2007), 93–112. https://www.academia.edu/627551/
Westerman, James S. The Osireion. http://jameswesterman.org