The Normal ending makes a point to reinforce the notion that you could have done more to save the various people you met on your journey, which is actually possible if you are willing to put in more effort. Here is a step-by-step guide on how to reach the Sacred Ground and the true final boss that is housed within:
Step 1: Once you come to the point in the labyrinth where Professor Booster falls from the sky, attempt to ignore him and your curiosity. Either make the very tricky jump or use the Level 3 machine gun to make it to the other side and continue your journey. If you do talk with Booster, then he will give you the jetpack 0.8 and die, but ignoring him will ensure his survival and grant you the 2.0 version later on.
Step 2: When travelling through the Labyrinth with Curly, there will come a point where she stands still in front of a broken robot. Ignore her for now and go the bottom-right corner to pick up the Tow Rope. After the boss-fight with the core, allow yourself to drown, so that Curly saves you. Now use the Tow Rope to carry the unconscious robot along with you.
Step 3: When carrying Curly through the Waterway, there will be an area with multiple tides pulling you in many directions with an exit in the upper-right corner. Get to that exit, but make sure you also emerge from the water and get to the door (this can be tricky). Once there, put down Curly on the bed and use the book on the bookshelf to drain the water from her. Pick her up again afterwards and continue through the Waterway.
Step 4: You will meet Curly again in the plantation where she is in a hidden section somewhere at the bottom. She will have Amnesia (again), so go to the graveyard in Mimiga Village to reach the section you couldn't get to earlier. Here you will find Ma Pignon, talk to him and go through his funky dialogue tree to get his badge. Check your inventory to learn that the badge is fake and talk to him again for a boss-fight. Now carry him to Curly for some extra plot.
Step 5: When you enter the Last Cave, it will be a more difficult version than the regular one. Your levels will be set to 1, so grind a little and continue onward. This may take a while.
Step 6: Fight the Doctor and Misery as you usually would, but during the escape enter the small shack. Save and the floor will burst open, which leads you to the Sacred Ground.