Learn Finnish: Onkalo, jonne menemme, on tutkittava kompassilla. - The cavern where we are going must be explored with a compass.

This ties into the current lesson! In standard written Finnish, when referring back to a specific physical location noun (here, onkalo), you must use a form of the relative pronoun joka. jonne is a relative adverb derived from joka, meaning "to which" or "where to". While minne (derived from mikä) is very common in everyday speech, standard grammar prescribes a joka-based word here.

Could I use johon instead of jonne in this sentence?

Yes, absolutely. johon is the actual illative ("into which") case of the pronoun joka. jonne and johon are entirely interchangeable in this context, and both perfectly follow the rule of using a joka-based word for specific physical locations.

What kind of structure is on tutkittava?

This is the passive present participle (using the -ttava/-tävä ending) combined with the verb olla to express necessity. It translates literally to "is to-be-explored" or "must be explored". It functions as a slightly more formal, passive equivalent to using a standard necessity verb like täytyy.

Since necessive sentences often require a genitive subject, why is Onkalo in the nominative case here?