Hehe,

Alrighty here's some screenshots for you ;) 

The secret cavern can be found behind this rock! you can enter that cavern aaaaand,

This spot in the cavern is where Murks can be encountered. After you win the battle, leave the cavern and come back and the encounter can be reset. (3 different Murks can be fought here, entirely randomized)

Hope this helps! I love your questions <3