At least six people are feared dead and nine others have been injured after a tourist submarine sank off the coast of Hurghada in the Egyptian Red Sea early on Thursday, BBC reported. Confirming the incident, local sources told the publication that the vessel, known as “Sindbad,” was carrying around 40 passengers at the time of the accident.

According to the British broadcaster, the submarine sank near the harbour in Hurghada, a popular tourist destination. While 29 people were rescued from the sinking submarine, authorities reported that at least six passengers remained unaccounted for. Among the injured, four are said to be in critical condition and have been transferred to nearby hospitals for treatment.

The Sindbad submarine, which has been in operation for several years, is one of the few recreational submarines in the world. According to the company’s website, it is one of only 14 such submarines globally and is known for taking tourists 25 meters (82 feet) below the surface to explore coral reefs and marine life in the Red Sea. The submarine features 44 passenger seats, including two pilots’ seats, and a large round viewing window for each passenger to enjoy the underwater scenery.

Local authorities are investigating the cause of the sinking, and more details are awaited.