Learn Turkish: Çatlak derin mi derin, ama üstünden atlayabiliriz. - The crevasse is very deep, but we can jump over it.

Placing the question particle mi (or mı/mu/mü) between two identical adjectives is a common Turkish structure used to intensify the meaning. Instead of just saying 'çok derin' (very deep), derin mi derin emphasizes that it is incredibly deep, similar to saying 'deep as deep can be' in English.

Why does üstünden take the ablative (from) case ending -den here?

In Turkish, verbs that involve passing through, over, or across a physical space typically require the ablative case (-den/-dan). Because you are moving across the open space of the crevasse, you jump üstünden (literally 'from its top') rather than to it or at it.

Could we use the present continuous tense and say atlayabiliyoruz instead of the aorist atlayabiliriz?

You could, but atlayabiliriz sounds much more natural for this context. When expressing a general capability or proposing a practical solution to a problem, Turkish speakers prefer the aorist tense combined with the ability suffix (-abil).

Is the word çatlak only used for physical cracks like a crevasse?

In this sentence it means a physical crack or fissure in the ground, but it is also highly common casual slang. You will often hear çatlak used to describe a person who is crazy, eccentric, or a bit unhinged.