Another means by which sonar could be hurting cetaceans is a form of decompression sickness. This was first raised by necrological examinations of 14 beaked whales stranded in the Canary Islands. The stranding happened on 24 September 2002, close to the operating area of Neo Tapon (an international naval exercise) about four hours after the activation of mid-frequency sonar.[18] The team of scientists found acute tissue damage from gas-bubble lesions, which are indicative of decompression sickness.[18] The precise mechanism of how sonar causes bubble formation is not known.