Tusk!

It may seem a little odd to consider this song underrated - one that, quoth Wikipedia, "reached #8 on the U.S. charts, #6 in the U.K. and #3 in Australia and Canada" - but as the song that lent this site it's name, it's a natural start - plus it's definitely strange, weird and arguably freakish.
By 1979, Fleetwood Mac, having been re-energized by the addition of Lindsey Buckingham (one of rock's true, if oft-kilter, geniuses) and Stevie Nicks, had released a couple of brilliant mainstream pop/rock albums (the one everyone knows: 1977's Rumours, and the one most people don't - the equally high-quality proto-Rumours 1975 eponymous album). But nothing really prepared folks for what was next. The band knew that trying to top the blockbuster success of Rumours would be impossible, so with the help of prodigious amounts of cocaine they (led by Lindsey) proceeded to craft this masterpiece.
The percussive, chanting title track did not sound much like anything else on the album, but as the debut single (but penultimate song) it definitely set its tone - one of feverish drug-fueled nervous paranoia. The bizarre accusatory lyrics, the nonsensical background vocals, the brass - oh, that brass. None other than the Pride of Troy - the USC Marching Band. It must have been a trip to hear this on the radio airwaves in 1979. Fleetwood Mac were exactly the type of bloated rock stars that punk was railing against, but ironically this album had such extreme elements of the avant-garde (to go along with some masterful classic rock performances and sweet pop, yet all tempered by Lindsey's flat-out weirdness) that this was as strange as pop music gets.
Recommended reading:
Stephen Thomas Erlewine's review of the album.
Recommended viewing:
the video, featuring the USC Marching Band -
Fleetwood Mac - Tusk (Original Video)


