Add an entry to every element in a nested data structure I currently use a data structure like this: 

(def sample-board 
  [[{:mine true} {:warn 2   } {:warn 1} {          }]
   [{:warn 2   } {:mine true} {:warn 1} {          }]
   [{:warn 1   } {:warn 1   } {:warn 2} {:warn 1   }]
   [{          } {          } {:warn 1} {:mine true}]])


Now I want to create a copy of the first structure and each element should be supplemented with :explored true. 

My approach so far looks like this: 

(def sample-board-solved
  (map (partial map 
                (partial conj {:explored true})) 
       sample-board))


I don't find the approach very appealing though. I thought about using for instead or maybe recur in combination with update-in but I did not bring it to work yet. 

I'd be glad on suggestions how to improve the code.
You could maybe make your solution a bit neater by using a lambda in place of one of your partials.  assoc might be a little clearer in purpose than a conj as well:

(def sample-board-solved
  (map (partial map #(assoc % :explored true))
       sample-board))


Using map like this will return things in sequences rather than vectors like the input data.  If it's particularly important that you get vectors out you can always use mapv