On Youtube there is one man, one legend that posts some of the best underground thrash you will ever hear. It's all recent stuff so don't expect ealry underground stuff. He post from anywhere in the 2010s to now.
Some recommendations would be Hazzard, Terrifier, sweeping death, and Mutard!
also my youtube channel has a massive playlist of metal here is the link: https://www.youtube.com/playlist?list=PLpMVFhzu663ECyx7nULcAX0hoiZzr1PfX
Here is his youtube link, and go subscribe: https://www.youtube.com/channel/UCD5Ny_jQ8cs9JXVPWXg9iNw