The hikers are climbing up the mountains.
De wandelaars klimmen de bergen op.
The hikers are descending down the mountains.
De wandelaars dalen de bergen af.
Hikers climbing up the mountains are moving in the opposite direction of hikers descending the mountains.
Wandelaars die de bergen opklimmen bewegen in de tegenovergestelde richting van wandelaars die de bergen afdalen.