A lava bomb hit a tour boat on Monday morning in Hawaii, injuring 23 people, the Hawaii County Fire Department said.

The lava punctured the boat’s roof and it returned to Wailoa Harbor, the fire department said.

Hilo Medical Center confirmed 13 passengers were treated at the hospital, and another 10 passengers with superficial injuries were treated on arrival at Wailoa Harbor.

Of the 13 injured passengers, four were taken by ambulance to Hilo Medical Center, according to the Hawaii County Fire Department.

One woman in her 20s was in serious condition with a fractured femur, the fire department said. Three passengers were in stable condition.

The lava bomb, or flying chunk of molten rock, left a large hole in the boat’s roof, the fire department said. A railing was also damaged.

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 It was unclear exactly where or when the incident occurred. But from where lava is hitting the ocean to the harbor is about an hour’s boat ride, depending on waves. 

Hawaii’s Kilauea volcano erupted in early May, sending a smoldering flow of lava into residential areas on the Big Island. Kilauea was still erupting lava as of Sunday, according to the United States Geological Survey.

A witness who saw the boat return said there were rocks all over it, and some people leaving the boat had burns and gashes on their legs. One person was taken off on a stretcher.

The boat reportedly holds 49 people, but it was unknown how many people were on board.

CNN’s Scott McLean and Sara Weisfeldt contributed to this report.

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