The Human Extinction League (abbreviated HEL) was an organization in early 21st century Earth. The HEL claimed Humans were gradually destroying the Earth and called for Humanity's voluntary extermination of itself, to the extent that members even abstained from sex. In line with their beliefs, the HEL vowed to stop NASA from spreading into the rest of the Sol system.
There were apparently various factions of the HEL in existence. While she was at college, Tera O'Herlihy, daughter of NASA scientist Dr. Marcus O'Herlihy, spent time with some of their less extremist members, as she enjoyed debating with the group even if she disagreed with their philosophies.
In 2020, the crew of the USS Lewis & Clark suspected Zoe Querez, who stowed away aboard the Lewis & Clark, of being a member of the HEL. Later in the mission it was discovered that Tera had been abducted by the HEL to force her father, who was part of the crew, to destroy the Lewis & Clark.
Marcus attempted to do so, but was stopped by the 23rd century Starfleet captain James T. Kirk, who had been transferred into the body of Shaun Christopher, the mission captain, by an alien probe. Once restored to his original body, Kirk then took the USS Enterprise- which had travelled back in time from 2270 to recover him - to Earth where he rescued Tera and turned the HEL over to the authorities via an anonymous tip after the Enterprise stunned the entire compound from orbit, effectively destroying the HEL for good. Doctor McCoy briefly expressed concern that their actions had changed history, but Spock assured him that historical records confirmed that the HEL had quickly fallen apart at this point in history and had little impact even when they were active, being nothing more than a historical blip in the future. (TOS novel: The Rings of Time)