Fishlegs' Ode to Dark Deep is a little song Fishlegs comes up with on the flight to Dark Deep in the episode "Quake, Rattle and Roll". He's so thrilled to find the ancestral home of Gronckles, including his dragon Meatlug, he just can't contain his excitement, much to the audible discomfort of both Hiccup and Toothless, who accompany them in exploring the island. Hiccup however insures Fishlegs that he loves the song once he stops singing.
"Dark Deep, at last I found you
"Dark Deep, the Gronckles surround you!"
"Dark Deep is your home
"Where the rock eaters roam
"Gronckles have fun
"Eating rocks by the ton!"
'"And the rocks are all rocky
"With families of rocks...
"...And their feeding of their..."
"I'll work on that last part, Hiccup."