They stood at the edge of a horizontal passage that sloped gently downward from the bottom of Luweng Ombo.
An underground river flowed through its center, reflecting the beams of their headlamps as broken white lines between the rocks. The passage was wide enough to walk through, but its incline made the wet surface feel like a road that was constantly threatening to send them somewhere lower.
In the distance, the roar of water grew louder.
“According to the old records, this flow heads downstream,” Oscar said, unfolding a waterproof map. “There’s a large weir that forms a rapid, then ends in a well-like pool.”
Ucok shone his light over the river’s surface. “Then we’d better not end up in that pool too.”
“The records don’t say anyone ever fell in.”
“They don’t say anyone managed to get out after falling in, either.”
Senja did not respond. She studied the walls on either side of the passage. Some sections were covered in a thin layer of moss that glittered when the light touched it. Something moved quickly in a crack between the rocks, then vanished. Maybe a cave shrimp. Maybe a small fish carried in by the current from deeper inside.
Above them, bats shifted along the ceiling. Their wings rustled softly, making the cold air seem to breathe with them.
Luweng Ombo was not empty.
During their research, they had found records of the cave dating back to the beginning of modern exploration in the 1980s. A joint expedition in the 1990s mapped roughly seven kilometers of underground passages. More recent records added hundreds of meters of new routes, including a survey conducted in April 2025 that discovered an additional passage approximately three hundred fifty meters long.
With a total length of more than seven and a half kilometers, Luweng Ombo was listed as one of the longest cave systems in Indonesia.
But numbers had never been enough to describe the place.
A map could show only distance, direction, elevation, and marks made by human hands. It could not show the cold creeping in through their gloves. It could not record the moisture clinging to their skin at nearly one hundred percent humidity. It could not reveal how the sound of dripping water could resemble the footsteps of someone walking far behind them.
The temperature at the bottom of the cave ranged from eighteen to twenty-two degrees Celsius. On the surface in Pacitan, especially during the dry season, the air could feel hot and dry. But at a depth of two hundred forty-six meters, the world had weather of its own. Air was trapped between the rocks, the river carried cold from deeper passages, and seepage kept the humidity close to one hundred percent.
Oscar folded his map.
“Downstream, we might find a bird’s nest deep inside,” he said. “It’s not impossible. Some chambers in this system connect to narrow fissures that can’t be seen from the main passage. There’s also an underground waterfall about seventeen meters high.”
“Have you seen it?” Ucok asked.
“Not yet.”
“Then don’t talk like you have.”
“I’m presenting data.”
“Data isn’t experience.”
“Experience isn’t always more accurate than data.”
“Then you can ask the waterfall later.”
Senja raised one hand. Their conversation stopped.
The three short knocks followed by one long knock they had heard from the passage before they moved were no longer audible. There was no reply. No falling rocks. Even the bats above them seemed to have chosen silence.
They had already understood one thing: something at the bottom of the cave knew they had arrived.
There was no evidence they could bring to the surface. No shape that could be photographed. Only sounds too orderly to be dismissed as coincidence, and a passage that appeared on neither the old map nor the new one.
But they had not come to challenge the place.
Good intentions, Senja thought, should lead someone to a good place and produce something good in turn. The belief might sound simple. Yet amid darkness that knew no witnesses, simple faith was often the only way to keep moving without letting fear make the decisions.
“Same formation,” Senja said. “Oscar in front. Ucok in the middle. I’ll bring up the rear.”
“Why am I in front?” Oscar asked.
“Because you talk the most. If there’s something ahead, it’ll hear you first.”
Oscar looked at her. “I’d like a more professional explanation.”
“Because you know the route from the map best.”
“The second explanation is better.”
Ucok tightened his backpack straps. “Then I’ll stay in the middle, so if Mas Oscar gets lost and Mas Senja quietly disappears, I can still choose which one to follow.”
Senja turned around. “I’m not going to disappear.”
“Good, Mas. I’m not ready to lead.”
They started walking.
Oscar led along the left side of the river, choosing footholds that were not too slippery. Ucok kept a few steps behind him, while Senja brought up the rear. They were never more than a few meters apart. In a place like that, too much distance meant losing each other’s voices; too little meant one mistake could send the entire group into the watercourse.
Their lights moved as three tiny points in the darkness.
Now and then, Oscar stopped to mark a fork with white chalk. He did not make large marks. Just a short line at knee height, visible enough when they returned without marring the rock surface any more than necessary.
“There are two routes here,” he said.
Senja moved closer. In front of them, the main passage followed the river downhill. A narrow gap opened to the right, sloping upward toward a darker chamber. The air from the gap felt colder, carrying the smell of old earth and something faintly like wet metal.
Oscar aimed his light inside.
“The route on the right isn’t on the map.”
Ucok touched the wall near the gap. “There are scratch marks.”
Senja shone her light over the rock. Beneath the layer of moss, three fresh parallel lines were visible, too straight to have been made by water. Beneath them was a brownish stain, like rust seeping from metal.
“Don’t go in,” Senja said.
Oscar looked at her. “We came to inspect the unmapped points.”
“We came to follow the water system first.”
“It might connect to the water system.”
“Maybe.”
“And it might be the clue we’re looking for.”
Senja stared at the gap. Nothing could be heard from within. That silence was precisely what made her hold back. Passages alive with water usually left sounds behind: dripping, flowing, or currents scraping over the rock at the bottom. The gap had none of those.
“We’ll document the right-hand route first,” she said. “We’ll continue following the river. If conditions allow, we’ll come back.”
Oscar looked ready to argue, but Ucok patted him on the shoulder first.
“The decision’s been made, Mas.”
Oscar sighed. “I just want it noted that I disagree.”
“It’s been noted since we set out,” Senja said.
They continued on.
The passage grew steeper. Water that had initially been only ankle-deep now rushed more strongly between the large rocks. In some sections, they had to move from one boulder to the next to avoid stepping into holes concealed beneath black water.
A pale fish shot between Ucok’s shoes. It had no distinct color, only a faint shadow that vanished before his light could follow it.
“At least there’s someone who likes having their picture taken even less than Oscar,” Ucok said.
“I don’t mind being photographed,” Oscar replied. “I mind when the results are bad.”
“All the pictures turn out bad here, Mas. It’s dark.”
“That’s exactly what makes it challenging.”
Senja smiled faintly. Their humor sounded ordinary, but she knew its true purpose. Oscar and Ucok’s voices kept them connected to the world they knew. As long as they could still tease each other, the place had not completely taken control of their minds.
The roar ahead changed into a pounding rumble.
They reached the edge of a wider chamber. The river dropped over a series of rock ledges, forming a short rapid whose water turned white before going black again. Beyond it, the passage descended toward a large pool whose surface reflected no light.
“A weir,” Oscar said quietly.
Senja shone her light to the right. The chamber wall rose high overhead, vanishing into darkness. Above them, the sound of flapping wings echoed from some alcove. On the left wall, moss grew thicker around the seepage, forming patterns like green veins.
Ucok crouched at the edge of the river. “There’s something in the water.”
His light revealed several small shapes moving against the current. Pale shrimp hid behind the gravel, while an eel coiled beneath a dark rock.
“The ecosystem is still active,” Senja said.
“And the pool?” Oscar asked.
Senja directed her light toward the pool at the end of the passage. Its surface remained black. There were no ripples, even though the river continued flowing into it.
At the edge of the pool, something that was not a rock came into view.
A long, pale object was caught between two boulders. Senja took one step down, then stopped when its shape became clearer.
It was a piece of rope.
Its fibers had blackened with water and age. Yet one end still held a knot tied using human technique. Near the knot hung a small octagonal piece of metal.
Oscar stared at the object without blinking.
“This isn’t our equipment.”
Ucok slowly rose. “It’s not rope used by modern teams, either, right?”
Senja did not answer immediately. She took out the waterproof camera and photographed the object from several angles without touching it.
Beneath the sound of the rapid, an extremely soft noise came from the direction of the pool.
Knock.
All three of them froze.
Knock. Knock. Knock—
Then one long, deep knock echoed from beyond the pool.
Oscar took half a step back. “That’s the same code.”
Senja switched off the extra light. “Don’t answer.”
“What if it’s calling us?”
“Don’t answer.”
Darkness closed around them more tightly.
A moment later, a single ring of ripples appeared on the pool’s surface. Then a second. The water there was not flowing downstream.
It was moving upward.
Senja switched her headlamp back on. The light fell directly on the wall behind the pool. On the wet rock surface, clear lines slowly appeared, forming an eight-angled sun symbol.
The symbol was not carved.
It had formed from water seeping out from inside the rock.
And at its center, a carving resembling an eye began to open.