A radioisotope might work, though good luck convincing people to dump tonnes of radioactive marker into a hole that nobody knows where it goes. A pinger from an

A radioisotope might work, though good luck convincing people to dump tonnes of radioactive marker into a hole that nobody knows where it goes. A pinger from an aeroplane would do an alright job, but they are usually specified to work for a week which might be far longer than the period underwater.