Underground the way I see it is rap made by artists that don't cater to audiences looking for the radio to feed them hip hop, that don't follow trends or make formulaic bullshyt.
Artists with a core following they cater to who respect the craft of emceeing and the integrity of the art of hip hop music.
Artists today like Mach-Hommy, Roc Marciano, Black Milk, Griselda Records, Elzhi, Planet Asia, Freddie Gibbs, Curren$y, Billy Woods & Elusive, Blu, Action Bronson, Crimeapple, Estee Nack & Davino, Ka, Earl Sweatshirt, Vince Staples, Fly Anakin, Jean Grae, Quelle Chris, Danny Brown, Noname, Mayhem Lauren, Mick Jenkins, Murs, Rahiem Supreme, Smoke DZA, The God Fahim, Ankhle John, Bishop Neheru, Vast Aire, Hus The Kingpin, Willie The Kid, Elcamino, Gavlyn, Murs, Recognize Ali, etc. etc.
I could go on and on, but if you really love hip hop and aren't just looking for the days when Jay-Z and DMX were dominating Top 40, then I promise you'll be thoroughly satisfied digging through shyt from those artists I named.
99% have projects out within the past 12 months.