December 18, 2025 | 07:26 pm
TEMPO.CO, Jakarta - Indonesian netizens have been raving about the "crying mountain" phenomenon after video footage online went viral on social media. The videos show spontaneous waterfalls forming on the hillsides of Sembalun valley, Lombok, West Nusa Tenggara.
"In geological terms, this phenomenon is known as intermittent rivers, which form seasonally during the rainy season and dry up during the dry season," Mirzam Abdurrachman, a volcanologist from Bandung Institute of Technology (ITB), told Tempo on Thursday, December 18, 2025.
According to Mirzam, several factors contribute to this spectacle, such as the impermeable characteristics of the rocks. The Sembalun area is dominated by volcanic breccia and relatively impermeable lava, so only a small amount of rainwater gets absorbed into the rock pores.
Another factor is the high rainfall and land conversion on the hillside. "Without tree roots to hold water, rainwater flows directly down the hillside," he said.
While Mirzam did not consider the phenomenon dangerous, he said it signals potential land use change. Without meaningful efforts, this would eventually increase the risks for floods and landslides. "Similar phenomena could occur elsewhere," he said.
Meanwhile, Tantan Hidayat, the chair of the Environmental Geology Team at the Center for Groundwater and Environmental Geology (PATGTL) of the Geological Agency, said the waterfalls did occur after four days of continuous rain in the region. According to the Mount Rinjani National Park officers, this is a common phenomenon. "When the rain stops, the waterfalls disappear," he said.
Tantan said these waterfalls are intermittent rivers that fill up with water once it rains, due to the impermeable nature of the volcanic breccia and lava rocks. "The surface water continues to flow downwards toward the end of the intermittent river channels on the slopes, creating the appearance of waterfalls," he said.
The spontaneous waterfalls formed in Sembalun Bumbung Village on Telaga Hill, Amben Hill, Batu Nunggang Hill, and Long Hill, according to the Geological Agency. This natural event can occur during heavy, continuous rain and disappear when the weather clears up.
Read: Extreme Rain Spawns Sudden Waterfalls in Bali
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