First 3 letter indicate biggest thing in world

First 3 letter indicate biggest thing in world

Solve this, …if u r brilliant..??

Its a 6 letter word
first 3 letter indicate biggest thing in world.
2, 3, 4 letter indicate parts of human body.
3, 4, 5 letter indicate part of circle..
the whole word is the key of internet now.
??101% brain work

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First 3 letter indicate biggest thing in world