Although not as spot-on as cl-r’s good suggestions in that it doesn’t necessarily imply that the hidden part/danger is greater than the visible one, the English notion of “There’s more than meets the eye” does at least imply that there is something unseen and/or “beneath the surface” that should/must be considered.
(from Urban Dictionary)
Possible French translations of this notion might include:
« Il ne faut pas se fier aux apparences », which is a French proverb
(see here for an example of this proverb’s use in connection with the tip of an iceberg)
(from Reverso, Le Monde’s Dico-Citations, and Simon LaForet/Google Books' respectively)
« [Attention!!] Un train peut en cacher un autre », which is an SNCF safety warning that has undergone what Charlotte Schapira calls «proverbialisation» and is used figuratively in other situations where something, usually a danger, is hidden (see page 87), such as Dominique Lamari uses it here with "l'iceberg"
(from Reverso, Persée, and Google Books, respectively)