At least two large sinkholes - one the size of a minivan - have appeared on roads in Guangzhou's central business district since the rainy season began.
Nobody has been killed or injured, and the holes have been roped off to keep cars and people away, but geologists say they are a warning sign of potential geological disasters.
Dr Chen Yu, a researcher at the Chinese Academy of Sciences' Key Laboratory of Engineering Geomechanics, which specialises in studying geological disasters, said rain could be the most frequent trigger of geological disasters in southern China in the coming months.
Water molecules significantly reduced the stickiness, and thus the stability, of clay, which was the main substance making up the relatively soft surface layers in Guangdong, Chen said. The situation could deteriorate if the underground layers were not pure clay but a mixture of sand and rock that could easily be disturbed by construction activities such as subway tunnelling.
In the city's latest geological hazard survey, issued by the Guangzhou Municipal Land Resources and Housing Administrative Bureau, 26 areas have been identified as particularly susceptible to severe geological disasters.
Fifty-seven locations - including the second phase of Guangzhou Baiyun International Airport and three metro lines - are listed as 'focus prevention zones' because they are prone to geological hazards.
Sinking or cave-ins may occur unexpectedly in these areas, damaging surface structures and killing construction workers, the report stated. The report warned the management of the Guangzhou Metro that there could be surface collapses during tunnelling in the Tianhe, Panyu and Huadu districts.
It said the company must closely monitor the situation and take precautions to ensure the safety of buildings, roads and residents near the work sites.
The biggest threat is mudslides, and one of the most vulnerable areas is a school in Baiyun district. More than 700 students and teachers living in a dormitory building at Guangzhou International Economics College were at risk of a large-scale mudslide, according to the report.
An official with the geological hazards department of the city's Land and Housing Bureau refused to say whether 'mitigation efforts' had been taken at the school.
'That information is confidential, and we are not allowed to reveal it to the public without permission from higher authorities,' he said.
The government survey also found that a hill in Dagang village in Panyu district was prone to collapse, endangering 727 residents and pedestrians, though the means of calculating the people at risk was not disclosed.