Collins Dictionary gives the following definition of the word "Stalactite."Stalactite is a cylindrical mass of calcium carbonate hanging from the roof of a limestone cave: formed by precipitation from continually dripping water."
To take this photo I did not need to dig underground. To the contrary, I took an airborne ride up on one of most exposed facades on a prominent street of midtown Manhattan. This is the fifth floor of a relatively new residential building. The photo shows the underside of concrete balcony slab. All slabs in this building are saturated with rain water due to multiple reasons: wrong pitch of decks, poor design of balustrade posts (balustrade post penetration detail is usually overlooked by designers), deteriorated caulking and several other factors.
Water dissolves salts contained in decks and carries them until evaporation. No paint lasts long in these conditions. Note peeling and mildew growth. Several other fine examples were available, but this one was the most spectacular. I even checked with my tongue if it is salty - just kidding.

One important note: wrong pitch of balcony deck, which is one of main reasons of this condition, usually comes as a result of a desperate designer's attempts to produce a neat facade. Many designers believe that decks sloped over the code minimum 1/4" look ugly. Often reality of a construction site prevails and a deck is produced with an opposite slope. Also, slabs sag initially after installation and creeping continues later to develop the opposite slope from the drains located next to the walls. Many building codes allow 1/360 maximum rate of deflection, which simply counterbalances 1% slope and create light opposite pitch, enough for water ponding.