Can you counter a mana ability with a counterspell?

Mana abilities are a special category of abilities that resolve immediately without ever being placed on the stack. Because counterspells (and all spells/abilities that say "counter target spell or ability") can only target things on the stack, they have no way to interact with mana abilities at all.

A mana ability is defined under CR 605.1 as an activated or triggered ability that could produce mana, is not a loyalty ability, and meets certain other criteria. When you activate a mana ability, it resolves immediately and produces mana — there is simply no window for an opponent to respond with a counterspell (CR 605.3).

This is different from most activated abilities, which do go on the stack and can be countered by something like Stifle. Stifle specifically cannot target a mana ability, because mana abilities never enter the stack (CR 605.3b).

Concrete example: You tap a Forest to produce one green mana using its mana ability. Your opponent cannot cast Stifle or Disallow to counter this action — the mana is produced immediately and the Forest ability never appears on the stack for anything to target.

Note that some abilities look like mana abilities but are not — for example, if an ability has an additional effect beyond just producing mana (like drawing a card), it may use the stack normally and could then be countered. Always check whether the ability meets the CR 605.1 definition.