It was a sensation in 2016. A multi-purpose waterproof metal detector for standard and under water treasure hunting (up to 100 feet/3 meters deep). The operation frequency of the device is 13,8 kHz, it has 20-tone audio identification plus lots of useful options.
Here’s a review of Whites Coinmaster metal detector. The article contains information about how many operating frequencies the device has, what kind of coil it is equipped with, if the headphones are included into the device package set, how long it can work using one battery set, etc.