People used to go for these records all the time. Not only deep air but also deep records on mix. There is a book by Bret Gilliam called 'deep diving' from the 90s with some crazy stories in there. It's intersting but not something you wanna take many lessons from. ... it's pretty... mmmh, old school, like smoking with your kids in the car or drinking from lead coated bottles.509 feet on air? Or am I misunderstanding?
It's an interesting read. It's diving history.