The RAF said it was caused by F-35 Lightning aircraft training about 25 miles (40km) offshore. The RAF said the aircraft were "completing operational training" inside a "supersonic offshore range area". It said such events were usually "sufficiently far out to sea to not cause sonic booms to be heard... under normal environmental conditions". However, it added that "climatic conditions... may have been a factor" in it being heard so far inland. It said "any inconvenience caused to local residents is regretted". Https://www.bbc.co.uk/news/uk-england-cornwall-56014534
