While Metallica plan to celebrate their 30th anniversary as a band later this year, the number 20 looms large this summer. The metal mavens recently passed 20 million Facebook followers and today (Aug. 12) they celebrate the 20th anniversary of their landmark 1991 self-titled album, also known as the Black Album.

The Black Album came out on Aug. 12, 1991, and has gone on to sell more than 15 million copies in North America alone on the strength of such hits as 'Enter Sandman,' 'Sad But True' and 'Nothing Else Matters.' It catapulted Metallica from thrash-metal kings to mainstream-rock royalty.

Now, 20 years later, the band's popularity is as strong as ever. The group has more than 20 million Facebook followers, which only trails Linkin Park (33 Million) and the Beatles (20.5 million) among rock bands. The next closest rock acts are Green Day (18 million) and AC/DC (15.6 million).

Metallica will be headlining the next stop of the 'Big 4' show with Slayer, Anthrax and Megadeth at New York's Yankee Stadium on Sept. 14. In December, they'll mark their 30th anniversary as a band with a series of hometown club shows in San Francisco.

In celebration of the Black Album's 20th birthday, our buddies over at Ultimate Classic Rock are running an intriguing video feature titled 10 Best Cover Versions of Metallica's 'Enter Sandman.'

Congrats to Metallica for an incredible career that is far from over. Keep riding the lightning, boys!

Watch Metallica Perform 'Enter Sandman' in 1992