Dangerous waters

[Rhye]

It's my intention to block the flow of triremes exploring the world map at least up to a certain time.
For example, if we could place on the map the famous Pillars of Hercules, the Greeks wouldn't reach the Atlantic and from there even China.
A similar measure could be placed in South Africa, below Magellan, and in other famous "dangerous waters".
Given that it's not possible to place an ocean tile there as it is adjacent to land, do you have any idea on what kind of artificial tool (fake resource / unit / natural wonder etc) could be placed, made impassable, and later removed? Must be something that graphically makes sense. Could be like a darker spot into the sea.

 

[PiR]

Barbs aren't enough to prevent these long trip boats?

Seems Ice, Fallout, and Atoll are options for that. Probably Atoll will not prevent boats from passing through. A volcano could be an option too.

I'm trying to simulate these but game keeps crashing. Let's try to reboot.

 

[PiR]

So only Ice prevents units from moving into the tile natively.

Although Fallout seems a more appropriate visual effect for dangerous waters, Ice looks OK to me for temporarily blocked tile, as a low-effort solution.

 

[Rhye]

Problem solved

 

[PiR]

Perfect!