My planet has interesting geography; there are large islands, each with a unique civilisation of people. But separating them are impassible barriers; they consist of a double range of tall, steep mountains, and between them a sea of "dead water".
These barriers were put into place to separate the islands and their civilisations, until the invention of flight. The mountains are smooth and steep, and made of hard rock; so very hard to scale (they are 3 to 5 km high). And once you get to the other side, you have a big, wide sea, of dead water. If it were normal water it would take several days to cross (them a couple hundred km wide) but they are filled with dead water; which is a substance that is made to be really, really hard to pass on a boat. It has the following properties:
- It is toxic to every known form of life. It doesn't necessarily have to hurt the skin, but ingesting it is (eventually) fatal.
- It is chemically stable over a time period of tens of thousands of years. Millions of years is not required; this was made by gods in order to separate their test subject civilisations. The first civilisation to cross the barrier wins!
- The stability requirement also entails that it either does not evaporate under regular weather conditions, or it does so by breaking down somehow into inert elements. What I want to prevent is clouds of this dead water forming and raining down on the surrounding islands, thus spreading its toxicity and killing off all life. It should remain confined to the "canals".
- Furthermore, it should somehow survive rainwater, which would land in these canals but be unable to leave as the mountains are in the way. Tall mountains do produce a rain shadow but we're talking about tens of thousands of years here; I imagine that that would accumulate over time.
Is there any real-life chemical substance that has all these properties? If not, how close can I get?
Also note that the experiment setting entails that real laws of physics govern every aspect of the planet. The materials do not have to be naturally occurring in the quantities I require, but they should be able to survive on their own, without any outside intervention whatsoever.