KOELSCHIP (OR COOLSHIP): BACKGROUND
Hundreds of years ago, there were no cooling systems as fast as the ones we have today to lower the temperature of the wort before starting the fermentation process.
In fact, one of the most effective ways to carry out this task was to transfer the hot wort to large, shallow, open containers in which it was allowed to cool slowly (Source: Hornsey, Ian SA History of beer and brewing. London: Royal Society of Chemistry, 2003)
Those open tanks or cooling "ships", better known as Koelschip or Coolship, besides serving as a method to cool and aerate the wort, were completely exposed to the air, and consequently, subject to microbial contamination, also known as the miracle of the natural inoculation of the yeasts and of the microflora (and even of the microfauna!) that inhabits the atmosphere.
SPONTANEOUS FERMENTATION BEERS TODAY
This type of systems are still used today when producing traditional lambic beers, but not only for the classic Belgian brewers such as Cantillon or 3 Fonteinen among others, but also for other much more modern ones such as North American The Veil, Jester King Brewery or Allagash Brewing Company which have adopted this type of practice for the creation of their recipes inspired by traditional lambic beers.
WHEN BACTERIAS AND YEASTS JOIN THE PARTY
One of the best ways to describe in a simple way what happens in those tanks is found in an article by Cat Wolinski for Vinepair, where she* explains how wild bacteria and yeasts that float in the air, jump into a pool and the party is ready. Of course, the theme of each pool party afterwards usually depends on several factors, such as the time of year, location, weather, etc.
These factors give each beer a series of unique and differentiated elements, typical of its particular "terroir". And from there the wonder that we know as spontaneous fermentation happens.