One said he could if he had two mana, so I Snapped his Birgi and untapped two of his lands, which let him use Tainted Pact to find Force of Will, incidentally exiling many important cards from his deck, and getting another card out of his hand to stop the other player from winning, which, in that spot, was far better for me than if Id been able to counter the Underworld Breach myself
Normally, players would get a chance to respond before the ability resolves, but Doubling Cube's ability is a mana ability, so 605.3b says An activated mana ability doesnt go on the stack, so it cant be targeted, countered, or otherwise responded to
Any "as [this Aura] enters the battlefield" or "[this Aura] enters the battlefield with" abilities of the Aura will also work
Japanese booster packs also offer different drop rates, which may improve your odds of finding an ultra-rare card