Bell Pits - a commonly used explanation for holes in the ground that come from nowhere-ish. They don't happen very far from habitation.

The pits on the second image appear to be sink holes, which appear when there's underlying limestone geology. Acidic water dissolves the rock, over hundreds of years, and eventually there's a cave-in. They show up in groups, often following a fault or underground ?

But we've speculated too far already, without doing a 'ground truth' survey.