In the early days of the cello quartet Apocalyptica, the four musicians played covers of heavy metal bands as an experimental joke.

"It was just a way to please our friends at music school," said cellist Perttu Kivilaakso during a phone call from his home in Helsinki, Finland. "It was a nice accident that it caught on. There was never a time when we felt we needed to bring classical and heavy metal together. Because to us, the emotions that we feel when we listen to certain heavy metal songs can also be felt when we listen to certain classical pieces.

"Music is about emotions. No matter what style, they can cause you to feel the same moods."

However, thanks to Metallica's popularity, Apocalyptica caught the attention of Mercury Records.

"The sad thing was the fact that I was only 16 at the time," said Kivilaakso. "So we all felt that I needed to complete my studies before I signed a record contract."

So his replacement, Antero Manninen became part of the line-up, and Kivilaakso continued his studies at the prestigious Sibelius Academy in Helsinki.

A year later, the quartet released its debut album, "Plays Metallica by Four Cellos," on which the cellists play eight dirgy, albeit loyal, Metallica covers, including "Enter Sandman," "Master of Puppets" and "Creeping Death," which was received with critical acclaim. Two years (and another album) later, Manninen left to join an orchestra and Kivilaakso rejoined the group.

"I came back in time for the album 'Cult,"' said Kivilaakso, who started playing cello when he was 5 years old. "And I noticed, as well as the group, that we were something of pioneers. There has never been a cello rock group before us. So there were no rules to follow. We basically were writing our own rules as we continued our musical journey."

With "Cult," Apocalyptica found itself moving further away from metal covers, something had continued on the second album "Inquisition Symphony," which featured covers of Pantera's "Domination," Faith No More's "From Out of Nowhere" and Metallica's "For Whom the Bell Tolls," "One," "Fade to Black" and "Nothing Else Matters."

"We were at a crossroads," said Kivilaakso. "Our record company wanted us to, yet, another album of covers. But we had different ideas. We wanted to play music for ourselves, and we wanted to play our own music.

"So we fired our record label and recorded the songs."

Out of the 13 songs, only three are so-called covers — Grieg's "Hall of the Mountain King" and Metallica's "Until It Sleeps" and "Fight Fire with Fire."

The group's most recent album, "Apocalyptica," saw Lombardo as a returning guest and new guests Lauri Ylonen and HIM's Ville Valo.