Learn Finnish: Onko tuo maailman vaarallisin kallio? - Is that the most dangerous cliff in the world?

When an adjective ends in -nen, you drop the -nen and replace it with -s- before adding the superlative suffix -in. Therefore, the stem vaaralli- + -s- + -in becomes vaarallisin (the most dangerous).

Why does the Finnish sentence say maailman (the world's) instead of using a location word for "in the world"?

With superlatives, Finnish commonly uses the genitive case (adding -n for possession) rather than a location case like "in the world". The sentence literally translates to "the world's most dangerous cliff". You will see this pattern often, such as in Suomen paras (the best in Finland / Finland's best).

Why doesn't the adjective vaarallisin have an -n at the end to match maailman?

Adjectives must match the noun they describe. Here, vaarallisin describes the cliff (kallio), which is in the basic dictionary form (the nominative case). Maailman is just an independent word saying whose cliff it is, so it doesn't force the words that come after it to take an -n.

Could I use se instead of tuo in this sentence?