To begin with, in order to unlock the secret levels, we need to open the game files.
This is done like this:
First, let's go to the Library.

Secondly, we find the game icon and click on it with RMB, then MANAGE, then BROWSE LOCAL FILES and click.

First, look for the levels folder

And go into it, scroll down and see the levels:

enemy_path_test
everything
sandbox
test
zecred
The method that I will tell you about right now can be used on other levels!
Open any of them. There we will see "stage": "-1".

Instead of -1, there can be another number or letter. If you opened from my list above, then there will be exactly -1.
Well, we replace this number with 0

And save the file. Restart or start the game.
Once you click "New game" you should have something like this appear on stage 0.
Now you can play the secret stages of Infinitode 2!
Please note that these stages DO NOT GIVE STARS!
There is ZERO ADVANTAGE for playing these levels!
These are just for fun, but you can farm the resources!

Try not to modify anything other than these files, if you have, you may have caused a bug somewhere. I am not responsible for mistakes made during this process.

But if you do not want to climb in the files several times. There is an easier second way,
First, instead of going into the levels folder, you need to go to the folder "res".

There we will see the "basic-level-stages" file,

Immediately we see the lines "name": "-1"

go into it and delete part of the code as in the photo and you're done!

And save the file. Restart or start the game.
Once you click "New game", and we see a new stage "-1", if you have not used method 1 yet, then there will be exactly 5 levels!

1) Everything - At this level, all boosters in the game are installed

2) Enemy_path_test - As I understand it, this is the level for testing the animation of one of the bosses, namely his body

3) Test - Normal level without oddities, just installed IN5-AR 10 level mining Infiar.And also at the start we are given 100,000 coins

4) Sandox - where 16 SCL1-R level 10 are installed, mining Scalar near them boosters for mining speed and experience. Inside the circle with these blocks is a level 10 Gauss with 2.64 million damage. Also there is a tower, sniper, cannon, frieze and other level 10 located near the path of the waves. This level is great for farming Scalar.

5) Zecred - The level is not clear to me.