An article in the Independent on Sunday (10 December 2006), headed “Why should the Iranians help? Here’s why” starts with the sentence “It is difficult to under-estimate the taboo which has been broken by the Iraq Study Group’s suggestion that the United States seeks a diplomatic engagement with Syria and more particularly Iran, in order to alleviate its deteriorating situation in Iraq.”
Putting to one side whether “seeks” or “seek” would be better here, the sentence opens with a phrase that means the opposite of what the writer intends to say.
If we under-estimate something, we do not pay as much attention to it as it deserves. Here the writer wants to say that the breaking of the taboo is such an important step that no matter how important we think it, it is actually even more important.
Two ways in which he could have expressed himself better are “It is difficult to over-estimate …” or “It is easy to under-estimate …”