To start there is a text bug error that others reported as soon as 1.19 released (I'll fix it for 1.20).
The achievement in question is the achievement for Diamond Dust Zone 1: you have to find the secret path under the main one, but you have to fully complete it, meaning that it won't count if you leave the underground area too soon.
You need a power character to access it easily (Knuckles, Bluckles or Wechnia).