Learn Farsi: آنها پوسته‌ی زمین را لایه لایه بررسی می‌کنند. - They examine the earth's crust layer by layer.

In Farsi, repeating a noun often creates an adverb describing how an action is performed. Here, لایه means 'layer', so the reduplication لایه لایه translates to 'layer by layer' or 'in layers'. This is a common pattern you will see in phrases like قدم قدم (step by step).

Why is there a ی at the end of پوسته (puste) before زمین (zamin)?

The word for crust or shell is پوسته, which ends in a silent 'h' (pronounced as 'e'). When linking it to زمین with an ezāfe to mean 'crust of the earth', a 'y' sound is inserted to bridge the vowels. In modern Farsi spelling, this is written by adding a ی to make it پوسته‌ی زمین (puste-ye zamin).

Why does the object marker را (râ) come after زمین (zamin) instead of right after پوسته (puste)?

The direct object marker را must follow the entire noun phrase that receives the action. The object here isn't just the crust, it is the whole phrase 'the earth's crust' (پوسته‌ی زمین). Therefore, را must sit at the very end of that linked phrase, right before the adverb.