The Osireion at the rear of the temple of Seti I at Abydos, the underground entry to the Osireion is at the top of the picture, see image below
The Osirion or Osireon is located at Abydos at the rear of the temple of Seti I. It is an integral part of Seti I's funeral complex and is built to resemble an 18th Dynasty Valley of the Kings tomb. It was discovered by archaeologists Flinders Petrie and Margaret Murray who were excavating the site in 1902-3. The Osirion was originally built at a considerably lower level than the foundations of the temple of Seti, who ruled from 1294 - 1279 BC. While there is disagreement as to its true age, Peter Brand says it "can be dated confidently to Seti's reign", despite the fact that it is situated at a lower depth than the structures nearby, that it features a very different architectural approach, and that it is frequently flooded with water which would have made carving it impossible had the water level been the same at the time of construction.