A tower of rocks suddenly broke off a mountain wall overlooking a ravine and collapsed onto several tour boats, pushing a huge wave over the lake in Capitolio, Minas Gerais state.
Videos posted on social media showed tourists screaming as a huge block of rocks hit the water. At least one boat sank after it wrecked, while other boats fled in search of escape.
Rescuers, including divers, said they were searching for 20 people believed to be missing.
The nine victims sustained broken bones and one of them was taken in serious condition to hospital with injuries to the head and face. Pedro Ihara, a spokesman for the fire department, told reporters that about 23 others were treated for minor injuries.
The area has been witnessing heavy rains for two weeks, which may have led to the loosening of the rocks.
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