I would recommend beating the game before attempting this, and you will need access to most of the map, as well as torpedoes and speed boosts.

To access hardcore mode, you must shoot three hidden crystals. This guide will show you how to find them and what to do afterwards.

Note: Crystals can be done in any order; I have just numbered them for simplicity’s sake.

This crystal requires some patience. It is located in ‘Seaweed Forest,’ in the room to the left of the room where you fight The Tranquil. In the top left corner of this room there is a giant stone slab.
Wait here for a minute. Eventually the slab will part and the floor beneath you will disappear. Down this path is the third crystal, which you should shoot.

Crystal 2 is located in ‘The Abyss.’ It is directly beneath the little cove that the station is in and in the wall to the left, which must be shot with torpedoes.
In this hidden room, there is an upgrade crate at the end. However, the path to the crystal is marked by some smaller crystals in the middle of the room. Go through the ceiling and you will find the second crystal.

Crystal 3 is located in ‘Pipe Central,’ in the room above the station, in a pipe to the far right. There, you will be able to go through the wall in the picture.
Here you be able to shoot the first crystal, and progress to the next one.

When you have shot the 3 crystals, you should go to the nearest station and activate it. Here you will notice a strange triangle on the map. You want to get to the center of this triangle.
The station closest to it is the South Capital Station. Travel there and go to the room to the left. You may recognize this as the room where you fought The Chain Gang. In the center of the room, there is a lighthouse. Go in front of its light.
Now, go to the left. There will be some blue floaty particles. Touch them and they will lead you to the unlock for hardcore mode!

This guide would not have been possible without the in-game detective work of:
Klaimen
hereticandy
Moham-Mad Hatta
Elfu
I only wrote down what they discovered in a guide. If you enjoyed this guide or found it helpful, please give it a good rating or like it. If there are any problems, feel free to comment and I will fix them to the best of my ability.