A projectile from the Kilauea volcano in Hawaii struck the roof of a sightseeing boat carrying people watching lava from the two-month-old eruption, injuring 23, the fire department said. Thirteen of them required hospitalization and the rest were treated at a harbor when the boat engaged in a "lava tour" returned to the Big Island, also known as Hawaii. One woman was in serious condition with a fractured thigh bone. She was transported to Honolulu for further treatment. The total number of people on the boat was not immediately known, the Hawaii County Fire Department said. The boat’s distance from shore and from the site of the lava explosion wasnt known as well, authorities said. In the early morning incident a "lava bomb" punctured the roof of the boat and damaged a railing, the department said. It gave no further details. "It was an explosion, basically," said Janet Snyder, a spokeswoman for the county mayor, told the Tribune Herald newspaper. "It punctured a hole right through the roof of the boat." She described the boat as "covered with lava." The eruption has destroyed hundreds of homes since it began on May 3.  Kilauea is one of the worlds most active volcanoes and one of five on the island. Lava flowing into the Pacific is a spectacular sight, producing a foggy haze known in Hawaii as "laze." The US Geological Survey says explosions of varying sizes happen whenever 2,000-degree lava enters much colder seawater. Some of those explosions can be so tiny they are hard to see. But when the conditions are just right, much larger explosions send molten rock and other debris high into the air, according to USGS geologist Janet Babb. Scientists believe that volcanic activity can be a precursor to a major eruption, similar to a Kilauea eruption in the mid-1920s.