A molten chunk of lava from the Kilauea volcano penetrated the roof of a commercial tour boat injuring 23 passengers on Monday.

The Lava Ocean Tours boat was hit by a basketball-size “volcano bomb” from a littoral explosion of lava hitting the salty seawater at the volcanic ocean entry point at Kapoho bay, reported USA Today. The boat, which had around 52 passengers onboard, returned to port an hour after the sudden eruption. 

The chunk left a hole in the roof of the boat operated by Shane Turpin and crashed into the seating area injuring the passengers. Debris from the eruption could be seen strewn around the seating area in videos pertaining to the incident.

"Of the injured, four were taken by ambulance, one seriously injured with a fractured femur," Hawaii County officials said to USA Today. "Most of the injured passengers had superficial injuries and were treated on arrival at Wailoa Harbor in Hilo."

13 people were treated at Hilo Medical Center in Hawaii of which nine drove themselves, a spokesperson for the medical center, Elena Cabatu, told Hawaii News Now reporter Mileka Lincoln. A 20-year-old woman was in a critical condition with an extensive pelvic/leg area injury. The other 12 suffered minor burns and were released after treatment.

In an Instagram upload, Lincoln shared a video of the incident taken by a man from California who was among the injured. “You can hear passengers screaming as the lava bomb lands on their tour boat,” she wrote in the post. 

Lava Ocean Tours is one of the few companies that offer daily outings to watch lava flow from the Kilauea volcano into the sea. They charge $220 for the excursions.

Turpin, the tour operator told the Guardian he and his tour group had been in the area for about 20 minutes making passes of the ocean entry about 0.28 miles offshore. Observing there weren’t any major explosions, he navigated his vessel to 0.14 miles away from the lava, which is when the incident happened.

Last year, Turpin was fined $15,000 for repeatedly launching lava tour boats without proper permits, according to reports.

Department of Land and Natural Resources (DLNR) officer Jason Redulla said a multi agency investigation that includes the U.S. Coast Guard, the Hawaii County police, and the DLNR is underway to identify the details of the incident.

The Kilauea volcano has continued to vigorously spew lava from an opening in the ground for the past two months, forcing evacuations in the area. When hot lava from the volcano hits the cold seawater, littoral explosions happen. Reports show that the lava entry point also produces hazardous gases that can damage the lungs.

Since May, when the volcano started erupting, mariners without explicit written permission from the Captain of the Port of Honolulu were required to keep a mandatory 0.18-mile distance from the lava entry point. Licensed lava boat tour operators could go as close as 0.06 miles, which was changed on July 11 to 0.03 miles. Redulla said this might be brought back to 0.18 miles after the recent incident.