September 16, 2026 | 05:13 pm
TEMPO.CO, Jakarta - Researchers at the National Research and Innovation Agency (BRIN) revealed an anomaly in sea level rise around Mount Anak Krakatau on Monday afternoon to evening, September 7, 2026. The timing coincided with the disappearance of a group of journalists in the waters of the Sunda Strait.
The irregular sea level data was captured by a tsunami monitoring sensor at the Inexpensive Device for Sea Level (IDSL) station, also known as the Pumma device, developed by Semeidi Husrin and his research team. The device was installed in the waters around Rakata Island and Sebesi Island, part of the Lampung region, following the 2018 eruption and deadly tsunami of Mount Anak Krakatau.
Semeidi explained that tidal patterns typically form a standard sinusoidal curve. On graphic readouts, those wave peaks and troughs usually appear pointed or rounded. However, from Monday afternoon through evening, the shape of the sea level wave was markedly different.
"It's as if the peak tide is cut off, so it's flat and has many significant wave ripples on top," the chief expert researcher at the Geological Disaster Research Center told Tempo on Wednesday, September 16, 2026.
An anomaly in sea level rise around Mount Anak Krakatau on September 7, 2026, detected by the Pumma device made by BRIN researchers. Doc: BRINThe anomaly data also drew the attention of his colleagues in Italy, leading to a joint discussion. "But it's clearly not a tsunami, the difference is very clear," Semeidi added.
Initial theories suggested the phenomenon stemmed from pockets of trapped gas or floating debris released after volcanic activity localized strictly around Mount Anak Krakatau. However, a similar wave pattern, albeit of weaker intensity, was also recorded by the Pumma station on Sebesi Island.
For now, Semeidi assessed that the sea level anomaly on September 7 was not localizedto the eruption center, but was instead widespread. Nevertheless, Semeidi and his team have been unable to reach a definitive conclusion due to a lack of supporting data.
He explained that during the event, the Pumma station's closed-circuit television (CCTV) camera was out of service. Additional limitations included the lack of localized weather sensors and satellite imagery obscured by fog. “Those are some of the obstacles that prevent us from drawing any conclusions about the phenomenon that occurred at that time,” he said.
Semeidi noted that he requires collaboration from external specialists, including volcanologists. “[We need to] see possibilities like cracks on Anak Krakatau that might allow seawater to enter and change its surface,” he said.
Regarding a viral video showing receding sea levels on September 6, which was linked to an eruption of Mount Anak Krakatau, Semeidi confirmed the information was false. On that day, data obtained by Pumma showed normal sea level patterns.
Athanasius Cipta, a mapping surveyor specialist from the Geological Agency, said continuous eruptions of Mount Anak Krakatau occurred from September 4 to 5. By the early morning of September 6, the Geological Agency declared that the continuous eruption phase had ended.
Meaning, he added, the sea recession on September 6, even if it occurred, "Is not directly related to the eruption of Mount Anak Krakatau."
Meanwhile, search and rescue (SAR) teams as of Wednesday morning, September 16, 2026, are continuing efforts to locate five journalists and three crew members who have been missing since September 7.
Read: Multiple Life Jackets Found During Search in Sunda Strait
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