No missable achievements (plus 23 unknown)
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Kill an acid antlion worker.
Help Griggs and Sheckley hold off the antlion invasion inside the mine shaft.
Defeat both antlion guards outside the White Forest.
Survive the Hunter ambush with Alyx.
Destroy the Combine Autogun in the junkyard.
Survive the ambush at White Forest Inn.
Secure the launch doors on missile silo 2.
Kill an enemy by planting a hopper mine.
Save the missile silo from the Combine offensive.
Unlock the rocket launcher lambda cache in chapter Under The Radar.
Find every radar cache in chapter Under The Radar.
Beat DOG in a race to the White Forest base.
Steal a Zombine's grenade.
Run over 20 enemies with the car in Episode Two.
Kill 30 enemies with thrown physics objects.
Kill a Hunter with its own flechettes.
Find and break every web cache in Episode Two.
Defeat the chopper in Episode Two without any misses.
Kill a Combine soldier with his own grenade.
Send the garden gnome into space.
If you are reading this achievement, Gabe Newell has successfully launched Gnome Chompski into space. If you did not also receive the achievement 'Manufacturing Ascent', Newell has abandoned his plans to shoot Noam Chomsky into space.
Save all buildings outside the missile silo from destruction.
Squish every antlion grub in Episode Two.