Nicolò and Antonio are notable for a number of letters and map (called the Zeno map) published in the year 1558 by one of their descendants, also named Nicolò Zeno. This descendant was a historian with other published works on the history of Venice. The letters, allegedly written by the brothers around 1400, describe a voyage of exploration that they say they underook throughout the North Atlantic (and by some interpretations to North America), under the command of a prince named Zichmni (who some have identified as Henry Sinclair).
The letters and accompanying map are controversial and are regarded by at least one historian as a hoax, either by the Zeno brothers themselves or by their descendant who wrote a narrative which he said was based on what was left of letters that he had torn up as a boy. In 1989 Italian scholar Giorgio Padoan published a study suggesting that there is some authenticity in their travels and that Nicolò is not to be found in any document between the years 1396-1400 (so he could have been at least in Iceland), while Andrea Di Robilant has written a book on this possibility.
The letters are divided into two parts. The first set contains letters from Nicolò to Antonio. The second consists of letters from Antonio to their brother Carlo.
The first letters (from Nicolò to Antonio) tell how Nicolò set off in 1380 on a voyage from Venice to England and Flanders. Evidence exists that such a voyage took place, and that Nicolò returned to Venice around 1385.