Learn Russian: Геологи пошли в тёмную пещеру, чтобы найти новый образец. - The geologists went to the dark cave to find a new sample.

Пошли is the perfective form, created by adding the prefix по- to the past tense of идти (to go by foot). With verbs of motion in the past tense, по- focuses on the start of the trip. It tells us the geologists set off or departed for the cave, completing the action of leaving, rather than just describing the ongoing process of walking (шли).

Why is it в тёмную пещеру and not в тёмной пещере?

Because the geologists are moving into a destination. Verbs of motion like пошли require the Accusative case to show direction. Тёмная пещера (dark cave) is feminine, so the endings change to -ую and -у. If they were already resting or working inside the cave, you would use the Prepositional case to show location (в тёмной пещере).

How does чтобы work with найти in this sentence?

The word чтобы means "in order to" or "so that". When the same subjects are doing the action in both halves of the sentence (the geologists went, and the geologists will find the sample), чтобы is followed by a verb in its infinitive (dictionary) form. Here, that verb is найти (to find).

Why doesn't the direct object новый образец change its ending?