Learn Dutch: De verslaggever ziet een ontzettend grote krater. - The reporter sees an incredibly big crater.

Here, ontzettend (incredibly) acts as an adverb of degree modifying the adjective grote (big). Adverbs do not get an -e ending. If it had an -e, it would be an adjective modifying the noun directly, which would mean 'an incredible, big crater' rather than 'an incredibly big crater'.

Why does grote get an -e ending here after een?

Krater is a de-word (de krater). When an adjective comes before a de-word, it always gets an -e ending, even when preceded by the indefinite article een.

Why does the base word groot change to grote with only one 'o'?

This is a standard Dutch spelling rule for keeping vowels long. In the base word groot, the double 'o' is needed to make a long vowel sound because the syllable ends in a consonant. When you add the -e ending, the word splits into syllables as gro-te. Since the 'o' is now at the end of an open syllable, a single 'o' naturally makes the long sound without needing to be doubled.

Does ontzettend always have a positive meaning like 'incredibly'?