Learn Swedish: Vidare upptäckte de fyndigheten under berggrunden. - Furthermore, they discovered the deposit under the bedrock.

Because vidare (furthermore) is placed at the very beginning of the sentence, it takes up the first position. Swedish is a V2 (verb-second) language, so the verb upptäckte must immediately follow it, pushing the subject de to the third position.

I've seen vidare used to mean 'further' or 'onward'. How can I tell it means 'furthermore' here?

When vidare is used as an adverb of motion or time, it usually comes after a verb, like in vi går vidare (we are moving on). When it is placed at the start of a sentence as an additive connector, as it is here, it acts as a formal way to say 'furthermore' or 'moreover'.

What kind of 'deposit' does fyndigheten refer to?

The word en fyndighet usually refers to a natural resource deposit, like minerals, ore, or oil. It comes from the root word ett fynd, which means 'a find' or 'a discovery'.

Why use berggrunden here instead of just berget?

While berget means 'the mountain' or 'the rock', berggrunden specifically means 'the bedrock'. It is a compound of berg (rock) and grund (foundation or ground), referring to the solid rock underlying loose surface materials like soil.