This is the 14732nd most frequent Russian word.
пещеру
"Cave."
In this sentence, 'пещеру' is in accusative case, indicating the direct object of the action, which is the cave being visited.
Мы посетили пещеру, чтобы увидеть её уникальные сталактиты.
We visited the cave to see its unique stalactites.
Here, 'пещеры' is in genitive case, indicating possession or relation to the phrase describing the bottom of the cave.
На дне пещеры обнаружили древние надписи.
Ancient inscriptions were discovered at the bottom of the cave.
In this context, 'пещере' is in dative case, showing the destination or direction implied in the phrase.