While the opponent can target a unconterable spell with a counterspell it will simply not do anything. In the same way you can try to destroy indestructible cards.Quote from Satori Grey »Thank you to both posters.
Is that a similar situation if I cast Obliterate, and my opponent casts mana leak?
It is not being countered per se, but instead just requiring a greater mana cost for the spell to be cast?
So you are offered the choice to pay for mana leak but not paying won't do anything.