People from around the world are being invited to take part in the 21st SPMK World Bog Snorkelling Championship.
The event takes place in the peat bog outside Britain's smallest town Llanwrtyd Wells on 28 August.
The winner of the contest is the fastest person to swim two lengths of a 60-yard trench filled with mud.
Anyone can enter, with a junior competition also held, and the only rule is that doggy paddle must be used.
Prizes are handed out to the overall champion, the fastest women, fastest child and best costume.
Currently the world record holder is Philip John of Bridgend who set the time of 1min 39sec for the course.
The idea for bog snorkelling originated 21 years ago after a pub chat in the Neuadd Arms Hotel in Llanwrtyd Wells turned to what to do with the garden which seemed to be nothing but bog.
© Adfero Ltd