Trujillo tells the No Treble Podcast: "The members of Metallica enjoy being creative. As they get a bit older and deeper in their careers, they get a little bit lax in terms of wanting to write songs.

"You'd be surprises how many known bands start writing with outside writers and stuff like that, and aren't actually writing their songs any more. With us it's the opposite - we've got so many song ideas, riffs, bass lines, whatever, that the hardest thing is trying to eliminate."

He describes the material as in the process of being "nurtured," adding: "It's pride; you're working on an art piece. It's got to be right, and what does that mean? Exploring.