Learn Finnish: Maanvyörymä tuhosi enemmän kuin mikään aiemmin. - The landslide destroyed more than anything previously.

While mikään normally requires a negative verb to mean 'nothing', it is also used in affirmative comparisons with kuin (than). In these structures, it translates to 'anything', emphasizing that the landslide was worse than all other things.

Could I use the partitive form mitään instead of mikään after kuin?

Not in this exact sentence. The word mikään acts as the subject of the implied second half of the comparison: 'more than anything (destroyed)'. Because its grammatical role is the subject, it must be in the nominative case (mikään). You would only use mitään if it were the object of the comparison.

What is the root of the word maanvyörymä?

It is a compound of maa (earth, land) and vyörymä (slide, avalanche). The noun vyörymä is formed from the verb vyöryä, which means to roll, surge, or cascade.

What is the difference between aiemmin and aiempi?

Aiempi is an adjective meaning 'earlier' or 'previous', which must modify a noun (like aiempi myrsky, 'a previous storm'). Aiemmin is the adverb form meaning 'previously' or 'earlier in time', which is used here to describe the timing of the destruction.