A technology that envelopes an underwater vessel in a large bubble, allowing it to travel at supersonic speeds underwater has attracted tremendous interest around the world - and caused some controversy, too. The Post last week reported that Chinese scientists had come up with an innovative approach to advance this so-called supercavitation technology, which is also being developed by several other countries. The advance involves applying a liquid additive to the vessel's surface, allowing the submarine to enter into the supercavitating state more easily and to be more easily controlled, said Li Fengchen, a professor at the Harbin Institute of Technology. Li spoke by telephone for about 20 minutes on the breakthrough to a Post reporter, who had identified himself as a journalist. But in the wake of the huge public interest generated by the story, Li has sought to distance himself. While the concept may sound incredible to many people, China is not alone in pursuing supercavitation technology. Russia, the US, Germany and even Iran have conducted tests of supercavitating vessels. The VA-111 Shkval torpedo, developed by the Soviet Union during the cold war, was the first underwater vessel to employ such technology and was capable of reaching speeds of more than 370km/h. German media in 2005 reported that researchers there had developed an underwater missile system named Barracuda, which allegedly could travel at speeds of up to 800km/h. The United States started developing the technology at least two decades ago. In 2011, a New Hampshire company, Juliet Marine Systems, announced it had developed the world's first prototype supercavitation boat that was capable of travelling both on and below the surface. Technology used in its prototype stealth boat, codenamed Ghost, allowed the vessel to reduce water friction by a factor of 900 and travel undetected underwater at speeds of up to 96km/h. Iran claimed it successfully tested its first supercavitation torpedo, named Hoot, in 2006. In theory, supercavitating vessels could reach the speed of sound underwater, or about 5,800km/h, according to the California Institute of Technology. But there are still many barriers to overcome, and no current technology allows vessels to get close to this top speed. While the advance by Chinese scientists may allow them to move a step closer to the holy grail of ultra-high-speed underwater travel, there are many technical problems to solve before they can make it a reality.