What caused an unexpected, record-setting tidal event on China’s east coast?

Late Monday evening, along the coast of northeastern China’s Bohai Sea, the water began to rise as it normally does when tide comes in.

But the water kept rising.

Under the cover of darkness, the tidal surge swept across several coastal regions in the provinces of Liaoning and Hebei, as well as the city of Tianjin, which is situated just over 100km (62 miles) southeast of the capital Beijing. The waters quickly flooded inland areas, prompting emergency responses and shattering official records.

“In the absence of obvious wind and waves, a sudden increase in water levels over such a large area has never been recorded either domestically or internationally,” said Fu Cifu, the head of the Storm Surge Forecasting Office at the National Marine Environmental Forecasting Centre, in an interview with state news agency Xinhua on Tuesday.

The event was strong and sustained, with water levels staying about 1 metre (39 inches) above normal for more than 20 hours, with several tide gauge stations in Liaoning reporting record-breaking levels.