Learn French: Les marchands sont descendus dans le gisement de saphirs. - The merchants went down into the sapphire deposit.

The verb descendre is one of the verbs of motion that takes être in the passé composé when it is intransitive (meaning it has no direct object). Because the merchants are physically moving themselves down into the deposit, we use être and make the past participle (descendus) agree with the plural subject. If they were taking something else down with them, you would use avoir instead, as in Ils ont descendu la boîte.

Why is it gisement de saphirs and not gisement des saphirs?

Here, de saphirs acts like an adjective describing what kind of deposit it is. When a noun modifies another noun to tell us its material or contents, French connects them with a simple de without the definite article. If you used des (which is de + les), it would mean "the deposit of the specific sapphires" rather than generally "a sapphire deposit".

Could we use sous or à instead of dans for going into a mine or deposit?

Dans is the most natural fit here because a gisement (deposit or mine) is viewed as an enclosed physical space or cavity. While you could say they went underground (sous terre), the phrase descendre dans specifically emphasizes climbing into the interior of that three-dimensional space.