Correct spelling, explanation: the word immense, similarly to many English words, originates in Latin immensus. The original word is built on the Latin suffix in-, which is also used in English, and the root word mensus, so just like in English, when the in- prefix meets a word beginning with m or n, the letter is doubled, so the spelling has to be immense.

Definition of immense:
adjective
1. immeasurable, great, large in scale
We found a peaceful clearing, there was just a lake, a forest, and immense silence all around.
I felt immense sadness when my father died.

Incorrect spelling, explanation: imennse is an incorrect word because it doesn’t make sense etymologically. You probably know a lot of words beginning with prefixes such as un- or in-, and you may have noticed that as a rule, if the letter of the main word is m or n, it’s doubled (like unnecessary or immeasurable). This works the same in the case of immense, but there is no reason to add the double n.

Incorrect spelling

Incorrect spelling

Incorrect spelling