there are many secret levels if you didnt guys didnt know about them, but you can still die in those levels
1) open the chest in the mines with the key. you get that blue eye icon on the top right section of the screen. Then in one of the jungle levels you will hear a sound and the eye icon will flicker, that means you are close to the black market entrance/secret level, the door will be inside some rocks so you need to move until the sound and flickering gets crazy and bomb that area. In the black market you can buy (or steal) for 50k the AHRK that is needed to go to hell
2) in the jungle sections, sometimes you see those pink bubbles in which enemies or the player get stuck for a couple seconds. If you throw yourself in there with a damsel and wait a couplek seconds, a big worm will show up and eat you and you get to the worm secret level
3) in the jungle section, sometimes you will have a level in which there are crypts of and stuff like that. there are 2 secrets here. First, bomb the ground below a gravetomb that says DASH. You'll find a shotgun there, and the 2nd secret, there is a tomb that has a skeleton with a golden crown, bomb below the crown, there is a door to another secret level
4) in the ice caves part, at some point you will see a big statue of a face with the ahrk simbol on its forehead. If you had the ahrk, let yourself die close to it, you will die and respawn inside the statue, pick the item that is there (jumping)
(this item is needed for the hell level)
5) in the temple part you need to get the item from the anubis boss (the one that flies) and you need to get a book or something lke that... im actually not so sure about this part since i never got the chance to get them lol