Locales/Ifurita's Tomb (The Jinnai Ruins)
This location appears in the El-Hazard OVA timeline and the Manga timeline.
Underneath Shinonome Prefectural High School, there lies a set of very Arabian-esque ruins. They are named after the man who 'discovered' them, that being Katsuhiko Jinnai. The previous statement is total bull as the ruins were actually found by a literature club freshman.
The room looks much like a type of tomb or possibly a type of small temple. The room is octagonal in shape and is littered with various symbols of myth and mysticism (the 'Star of David', the 'Yin and Yang', and some El-Hazard symbols like the prominent eye.)
In the center of the room's one wall is an ornate pillar which could appear to someone as a vertical coffin or some sort of alter. In reality it is a storage chamber in which Ifurita has slept under the school for some 10,000 years. When Makoto enters the ruins late at night, he activates the wake up sequence and she in turn sends him to El-Hazard.
The chamber was built for one reason, to keep Ifurita alive long enough so that she could complete the cycle that Makoto started. Most likely, when she deactivated the Eye of God and was blasted across dimensions she eventually found her way to Makoto's planet (recognizing it from his memories and his knowledge of the planet). However, she arrives some 10,000 years before Makoto will even be alive. Knowing that she is weak and must complete the temporal cycle, she constructs a device to keep her is a state of stasis until the time in which Makoto will stumble upon her. Most likely she employed the aid of the indigenous peoples of Japan at that time to help her construct the room, either by benign means or by using the superstitions of these ancient people to her advantage.
In the Manga and most likely in the OVA's, the tomb is discovered when a series of earthquakes (not uncommon for Japan) crack the ground and expose the long buried tomb, something that Ifurita had knowledge of through her connection with Makoto.
Now that Ifurita has been released and has both sent Makoto to El-Hazard and thus been reunited with Makoto, the tomb serves no other purpose except as an archeological curiosity (something for the archeology club to study perhaps)?