It's not stupid at all to say they are Underground. Metal in and of itself is Underground. If You are a Mainstream Radio fan, Metallica, Slipkno, FFDonkeyPunch, etc is as heavy as it gets.
In other parts of the world, yes Metal is not Underground, it's celebrated. But here in the states, the average radio listener thinks Metallica is the Devils music with satanic chords, and vocals you can't understand.
If you grew up with Metal, that's different, but Mind you, Metal as a whole is one of the least listened to genres of music in the States. Children Of Bodom and Amon Amarth are INDEED underground to people who don't listen to it. Just as whoever mid to club level country, indie and Hip Hop artists who are not on mainstream radio are HUGE to those who listen to them.
That Drummer Guy Presents:
Tuesdays and Thursdays from 5-9pm Eastern on http://www.metalnationradio.com