OK, I give up

Suddenly, a Rise in Piracy’s Price - NYTimes.com

On Tuesday, Somali pirates shot and killed four American hostages. A single hostage intentionally killed by these pirates had been almost unheard of; four dead was unprecedented.

WTF is almost unheard of? Does it mean unheard of but really not? Or is it more of an acoustical thing like, you know, a really really soft whisper almost unheard or heard, but in a really really loud place? I almost have no idea.