The 487-meter ascent to sacred Phnom Kulen in Siem Reap province is steep and rocky. At some point between the ninth and 13th centuries, a powerful leader decided to forge a clearer path.

The resulting 550-meter staircase made of rusty-red laterite stone has withstood generations of use, Khmer Rouge landmines and accidental battering from loggers.

Archaeologists are now working to understand who built the 15-meter-wide path known locally as Pleu Cere and why. At the moment, however, they know almost nothing.

Was the staircase constructed in the immediate aftermath of Jayavarman II’s 802 CE declaration of independence from the former empire of Java, when the king assembled a new mountaintop city of Mahendraparvata that is widely considered to be the start of the Khmer Empire?

Or did it come centuries later, after the capital had moved to Angkor Thom on the plains in the late 12th century, suggesting the mountain remained a regular path of pilgrimage to the sculpted caves and carved riverbeds locals continued to build?

So far, the only thing archaeologists know for sure was that the path was no small undertaking.

“If you deploy so much labor to build such a feature, it’s coming from the top,” said French archaeologist Jean-Baptiste Chevance, program director for the Archaeology & Development Foundation’s (ADF) Phnom Kulen program. “It’s not just some local rich guy who built it.”

For years, the heavy mining by Khmer Rouge forces, who used the mountain as a stronghold, prevented serious archaeological study.

More recently, Mr. Chevance and his colleagues at the ADF and Apsara Authority, which manages the Angkor Archaeological Park, have had other priorities, like documenting the sprawling city of Mahendraparvata using cutting-edge aerial laser technology known as Lidar that allows researchers to penetrate foliage for signs of human activity on the ground.

That pioneering research revealed a city of 30 temples and grids of dykes, roads and canals that had been swallowed by sediment and foliage over the past 12 centuries.

Now Mr. Chevance and his team are turning their attention to the staircase, which has been mostly cleared of landmines by the Cambodian Mine Action Center, and hope the use of Lidar and ground surveys will fill in information gaps.

“The problem is to know when it was built, because there are no carvings, no other remains,” Mr. Chevance said.

The staircase is interposed with four larger, flat constructions that appear to have functioned as rest areas with access to spring water, according to Im Sokrithy, deputy director at Apsara’s Angkor International Center of Research and Documentation.

In other places, builders appear to have carved the staircase directly into large boulders.

Mr. Sokrithy’s researchers interviewed nearby villagers and discovered that the path had become woven into local lore.

“They have been told about it from one century to another,” he said. “Their ancestors traveled on the road to the mountain along this way.”

The path is still used today, with loggers using it to haul wood down the mountain.

“They bring the trees down the foothills through the staircase,” Mr. Sokrithy said. “And then the tree damages some steps of the site.”

The deforestation has echoes with the past. Australian archaeologist Damian Evans, who collaborated with Mr. Chevance on past Lidar research, has speculated that ancient logging on the mountain ultimately may have hastened Angkor’s decline.

Mr. Sokrithy said that the Apsara Authority hopes one day to turn the staircase into a tourist attraction and is doing more research to see whether it is connected to a sprawling Angkorian road network.

For now, Mr. Chevance is only willing to say that the feature appears to be “quite carefully done—they chose somewhere where there was no cliffs, no rocks.”

Without further investigation, Mr. Chevance is loath to speculate on what he can’t see.

The staircase’s story is hard to ascertain, he said, “just by looking.”

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