The self-titled 1991 album from Metallica, also known as The Black Album, has been declared the top-selling album of the last 20 years, according to Nielsen SoundScan. The disc has sold more than 15 million copies in the U.S. alone during that span. Its closest competition comes from country superstar Shania Twain's 'Come on Over,' which lags behind by more than 200,000 in sales.

The self-titled album was Metallica's huge commercial breakthrough, spawning hits like 'Enter Sandman,' 'Nothing Else Matters' and 'The Unforgiven.' While seen at the time as too much of a departure and a bit of a "sell out" by the band's most hard-core fans, radio and the record-buying public at large ate it up and ultimately made Metallica the biggest heavy metal band of all time.

The Black Album was released just over 20 years ago, around the same time as Soundscan was put into place. In days of old, sales figures and reports to trade publications were mostly based on a random polling of key record stores throughout the United States. The information was then complied and condensed to get an overview of an album's sales and popularity. In 1991, however, the SoundScan system became the most common and most accurate method for tallying sales and has remained in widespread use since.

Interestingly, out of the top ten best selling albums from 1991 to 2011, Metallica is one of only four rock artists -- the Beatles ('1' at No. 5), Santana ('Supernatural' at No. 7) and Creed ('Human Clay' at No. 8) are the others. Meanwhile, Metallica come in at No. 4 when it comes to overall sales of the SoundScan era, clocking in just behind Mariah Carey.

So, who is the top-selling artist of the last 20 years: Pink Floyd? Nope! The Beatles? No, they're No. 2. The best selling artist of that time frame is -- drum roll -- Garth Brooks. Uhh, yeah. In answer to your obvious next question -- no, we're not sure how much of that was from his 'In The Life Of Chris Gaines' album.

Top Ten Selling Albums Of SoundScan Era (1991 - Jan. 1, 2012):

1. Metallica - 'Metallica' (15,735,000)
2. Shania Twain - 'Come On Over' (15,513,000)
3. Alanis Morissette - 'Jagged Little Pill' (14,714,000)
4. Backstreet Boys - 'Millennium' (12,168,000)
5. The Beatles - '1' (11,985,000)
6. Soundtrack - 'Bodyguard' (11,829,000)
7. Santana - 'Supernatural' (11,772,000)
8. Creed - 'Human Clay' (11,574,000)
9. N' Sync - 'No Strings Attached' (11,122,000)
10. Celine Dion - 'Falling Into You' (10,790,000)