Most of the season's key mysteries have been resolved by the end of True Detective: Night Country.
Danvers and Navarro learn that Annie K was killed in the lab after learning that the missing scientist Raymond Clark.....
.....has been holed up in a covert underground lab connected to the Tsalal research site.
After that, they make Clark reveal all of his secrets.
When Annie K was dating Clark, she found out that Tsalal was paying the mine to produce even more waste
Terrified by what they had done, Annie broke into the lab covertly and ruined years' worth of their work.
Even though he had promised Annie he would never injure her, it was Clark who finally completed the task by suffocating Annie.
But he says Hank (John Hawkes), the cop the mine sent to transfer her body, was the one who severed her tongue.