We're programmers. Overheard snatch of conversation between co-worker and boss (cleaned up):

Yes, we can certainly look into this new technology, but who knows what reefs await us?

After the call ended, we quizzed him on his use of reef in this way. He said that in Russian it is used as an idiom to represent potential hidden problems that are calamitous if encountered (pretty apt in the context, actually). I couldn't think of any English equivalent -- can you?

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    Making it hidden reefs could clarify the metaphor in English, approaching an island but not knowing whether a submerged coral reef would wreck the ship
    – Henry
    Commented Feb 25, 2016 at 19:56
  • What @Henry said. I must admit I thought I'd find more than one instance of What hidden reefs lie (in his course) in Google Books. I'd almost be tempted to describe this figurative usage as a cliche, but apparently it's not that common. Commented Feb 25, 2016 at 21:07
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    'Hidden reefs and shoals' is how I know it.
    – Dan
    Commented Feb 25, 2016 at 22:05
  • @Dan *shoals sounds like the closest English equivalent. You should post as an answer.
    – bib
    Commented Feb 25, 2016 at 22:37
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    Sounds like what I've heard described as "undocumented features."
    – Rob_Ster
    Commented Feb 26, 2016 at 1:04

16 Answers 16


How about "pitfall"?

a danger or problem that is hidden or not obvious at first

Yes, we can certainly look into this new technology, but who knows what pitfalls await us?


  • I agree with this - "minefield" has connotations of expected dangers, this has more of the feel of the original euphemism.
    – SeanR
    Commented Feb 26, 2016 at 12:03
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    This seems to hit the mark (and it certainly looks popular!)
    – CSJ
    Commented Feb 26, 2016 at 14:31

Mines and minefields are designed to be hidden dangers:

… but who knows what minefields lay ahead

(example of usage from ‘The Would-Be Daddy’ By Jacqueline Diamond via ‘Google Books)

With your natural/man-made comment/distinction in mind, quicksand is another hidden danger and like reefs, it usually occurs naturally and although potentially deadly, harbors no ill will towards those who unknowingly happen upon it.
(description of quicksand as being a hidden danger found in ‘Reading Minds: The Study of English in the Age of Cognitive Science’ by Mark Turner, via ‘Google Books’)

Although it could be used in an unanswerable question with “await/s,” similar to your colleague’s expression:

… but who knows if [patches of] quicksand await/s us?

... it would probably be more idiomatic if used in a declaration with “abounds”:

… but [as we all know] quicksand abounds [in this industry].

(example usage of “quicksand awaits” from ‘America's Unwritten Constitution: The Precedents and Principles We Live By’ by Akhil Reed Amar, via ‘Google Books’
and of “quicksand abounds” from ‘Katyń: The Untold Story of Stalin's Polish Massacre’ by Allen Paul, also via ‘Google Books’)

(It’s interesting to note that the short passage by Mr. Paul found at the last link contains not only my suggestion of “quicksand abounds,” but also two other options that are (or closely resemble) two other answers, both very good, imo)

  • We considered mines/minefields. There's a key difference: mines are, as you say, designed to be dangerous. Reefs are natural occurrences with no ill intent, but nevertheless deadly if poked the wrong way. Technology problems are not planted on purpose (as far as we know!)
    – CSJ
    Commented Feb 25, 2016 at 20:23
  • @CSJ The 'man-made and intended to harm' connotation is very slight nowadays (though perhaps not as slight as an 'evil polyp' connotation). But the 'resulting from man's ways of doing things' connotation (political minefield; tax minefield; legal minefield ...) persists. Commented Feb 25, 2016 at 20:44
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    +1 for mines and minefields. These days, mines tend to damage civilians they were never intended to harm more than anyone else.
    – DCShannon
    Commented Feb 26, 2016 at 2:40
  • @DCShannon Thanks and that’s a great point! I probably should have included “forgotten” somewhere to cover your point that the damage caused to unsuspecting victims of forgotten land mines/minefields left over from long-past conflicts is unintentional .
    – Papa Poule
    Commented Feb 26, 2016 at 14:56

Who knows what's going to crawl out of the woodwork. http://dictionary.cambridge.org/dictionary/english/come-crawl-out-of-the-woodwork


How about snag?

  1. A rough, sharp, or jagged protuberance, as:

a. A dead or partly dead tree that is still standing.

b. A tree or a part of a tree that is sunken in or protrudes above a body of water and is a danger to navigation.

  1. An unforeseen or hidden obstacle or difficulty: Our plans for the party have hit a snag.



The idiom is from sailing (more than from Russia). An American (car-centric) example would be 'pot-holes' or 'detours'.


Consider murky/uncharted waters,

a ​situation that is not well ​known and may be ​dangerous

[Cambridge Online Dictionaries]


Yes, we can certainly look into this new technology, but we could well be entering murky/uncharted waters.

Or if you are looking for a single word, the adjective dicey could be a potential candidate

involving a chance that something bad or unpleasant could happen (a dicey situation)



Yes, we can certainly look into this new technology, but we might end up in a dicey situation.


You could use gotcha, if it is not too colloquial and the programming context meaning is not too specific for you. I am a programmer and was not even aware of the latter.

  • Yes, gotcha was the first thing that came to my mind too. I know this was not specified in the question, but I think I was looking for physical world metaphors.
    – CSJ
    Commented Feb 25, 2016 at 20:50

I agree with pitfall, yet I'm going with a more naive approach here...

As alternatives to "reefs", you may use "dangers" or "risks". In particular "risks" sounds like "reefs" and implies a chance of happening or not. Or if you want to avoid giving them agency, you can use the word "obstacle". A similar word to consider is "hurdle".

I understand that these suggestions don't have the connotation of hidden. Yet, the phrase is telling that these dangers are not known ("who knows"). If you want to suggest that trying to forsee these dangers may not reveal them, you can add "hidden".


I ran into a similar situation some time ago and asked a similar question (SO EL&U). The best answer (I thought) was "Unknown unknowns" (Wikipedia).

That term has wormed its way into our company culture so much so that one of our engineers in Nova Scotia used it in a conference call the other day and he's thousands of miles away from this office.


The closest I can think of to reefs that is still somewhat idiomatic is "unexplored terrain". I got 5,570 hits in Google Books for that expression, most, it seems, would fit your meaning.


The OP is using 'reefs' to signal unknown dangers ahead. Reefs signify a much greater potential hazard if they are hidden.

Shoals are no less perilous than reefs being '... natural submerged ridges, banks, or bars that consist of, or are covered by, sand or other unconsolidated material, and rise from the bed of a body of water to near the surface. Often they refer to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation.'

In fact, the phrase I know like this - to signal unknown dangers ahead - is 'hidden reefs and shoals'. And I have just discovered that this is the title of the 18th in Alan Lewrie's Naval Adventure series!


We call them unknown unknowns.

This phrase comes from a speech by Donald Rumsfeld:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.


"Tip of the iceberg" is often used to represent a small visible piece with a large unknown piece under the surface.

(Posted as an answer since anon comments aren't allowed)

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    Welcome to English Language & Usage! This is a valid answer, no need to post as a comment. I'd recommend citing a source for it though, if you can. Commented Feb 25, 2016 at 23:46

A snake in the grass can sometimes be used to describe a concealed danger, but it is most often used to refer to a treacherous person.


"That could be a thorny path", as opposed to a "primrose path" (which is thornless and sweet-smelling). The metaphor comes from Shakespeare's Hamlet.


I think it can be paraphrased into a statement:

Yes, we can certainly look into this new technology, but the devil is going to be in the details.

A common saying which I hear is:

The devil's in the details.

fwiw: NY, USA

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