Here is the full list of all 15 Escape From Tethys achievements.
Defeat the rock creature in the caverns.
Put the rogue scientist to rest.
Destroy the reef guardians.
Defeat the IRI Assassin for the second time.
Destroy the missile frigate.
Defeat the IRI Assassin for the last time.
Finish the game on Easy Mode.
Finish the game on Normal Mode.
Finish the game in Normal Mode in under 45 minutes.
Find all the ammo upgrades.
Find all the health upgrades.
Find every upgrade.
Explore every area on Tethys.
Unlock the high-security gate.
Encounter the IRI Assassin.