Hidden gems

I had fun placing hidden references and homages in some levels of SOATE (can be through a level design or its title...). 

I like the idea that past and present are closely linked, and inspiration a never ending story of transformations. 

Will you find these references ? Will you find other ones I'm not even aware of ? 

(It is probably more easy for thoose who play the game, as level titles may give some clues, and because of light effect less visible on screenshots)

Hidden reference 1 :

Hidden reference 2 :

Hidden reference 3 : 

Hidden reference 4 :

Hidden reference 5 :

Hidden reference 6 :

Hidden reference 7 :

Hidden reference 8 :

Hidden reference 9 :

Have fun !