AP — Under a mountain on the outskirts of Jerusalem, workers are completing three years of labor on a massive subterranean necropolis comprising a mile (1.5 kilometers) of tunnels with sepulchers for interring the dead.

Up above, the Har Hamenuhot Cemetery dominates the hillside overlooking the main highway leading into Jerusalem from Tel Aviv. But in October, the cemetery’s management plans to open the first section of a sprawling catacomb complex which, when completed, will provide 23,000 grave sites for an increasingly crowded country.

“People will die probably forever,” said Arik Glazer, chief executive of Rolzur Tunneling, the company building the tunnel tombs, “so you have to get space for that.”

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Land is in short supply, and Jewish and Muslim burial customs require interring the dead in the ground and prohibit cremation. Tel Aviv and other increasingly crowded metropolises have embraced vertical cemetery structures to accommodate growing demand, but now Israel is looking for solutions below ground.

The hilltop cemetery on the edge of Jerusalem is almost at capacity, with nearly a quarter million graves. The first underground section opening in October will have capacity for 8,000. The remaining sections are slated to open in the coming years.

Even in the blazing summer heat, the labyrinthine vaults maintain their steady year-round temperature of 23 degrees Celsius (73 degrees Fahrenheit).

The limestone walls are lined four-high with tombs that resemble small Japanese capsule hotels. Giant flame-hued polyhedron light fixtures designed by German artist Yvelle Gabriel dangle at intersections between the avenues and streets deep in the mountain.

The entire project cost an estimated $50 million and took just over three years to complete. The tunnels take up just 5% of the total subterranean area of the mountain available for future tombs, Glazer said.

Part of the inspiration behind this project was the ancient Jewish custom of cave burials found at sites around the Holy Land, from the UNESCO heritage site of Beit She’arim near Haifa to rocky hillsides around Jerusalem.

“The basic blueprints for this project were the cemetery at Beit She’arim,” said Adi Alphandary, head of Rolzur’s business development. Those catacombs, active between the second and fourth centuries, were recognized by the United Nations as a World Heritage Site in 2015.

Israel Antiquities Authority archaeologist Amit Reem said that families would inter the deceased’s remains in the catacombs, then seal the door with a rock for eight months.

“When they opened the door of the cave, inside the cave was only the skeleton with no flesh,” Reem said.

The bones were then collected and often placed in stone boxes, known as ossuaries, inside the cave chamber.

While the modern-day burial chambers will simply be sealed with a grave marker, Hananya Shahor, executive director of the Jewish burial association in Jerusalem, said that Orthodox rabbis they consulted said the sprawling site is “100% acceptable according to Jewish tradition.”

“We are almost sure that people will like this way much, much more than the old systems of burial,” he said.