lol, how the heck does this happen?

[Armageddon1]

I moved trireme up from the south, found out it was a dead end, went to go back down and found that I was blocked by ocean tiles. Can tiles change or something? It's currently just sitting there, trapped.

EDIT: It looks like I did have an open borders agreement with Rome end, which must have put the trireme in the position. Disappointing, I was hoping for it to be some kind of prank from the ghost of Dennis Ritchie. Oh well, mystery solved, thanks for the input.

 

[Lordleoz]

Perhaps the Trireme was sailing in Roman territory but the open border agreement suddenly ended?

 

[Camikaze]

Odd. Worth noting that triremes can move in ocean tiles that are within your borders, but obviously that's not applicable here, as you're not Rome, and Roman territory doesn't extend to link the coastal waters.

 

[Armageddon1]

Perhaps the Trireme was sailing in Roman territory but the open border agreement suddenly ended?

There is that one open tile though that's not in that trapped territory, unless it somehow gives priority to a closer tile that may be in a non-accessible region? That must be what happened

Odd. Worth noting that triremes can move in ocean tiles that are within your borders, but obviously that's not applicable here, as you're not Rome, and Roman territory doesn't extend to link the coastal waters.

Right, I'll note for reference that I am America, who does not have this ability

 

[Lordleoz]

Closer tiles have higher priority but I noticed that certain tiles, such as the one to the south-west of Ottoman Caravel, are closer to the trapped territory than the untrapped fish tile.

 

[Louis XXIV]

There are a couple of Roman tiles where that coastal patch is the closest so it's not implausible that open borders ended and you popped there. Check your global history to see if you recently had open borders end. Maybe you didn't remember where your Trireme was, so it teleported.

 

[Halcyan2]

As others have noted, it could be due to a lapsed Open Borders agreement, especially if closer tiles (like that other fish tile) were occupied by other units at the time.

Others have also mentioned that Triremes can move into Ocean tiles within your cultural borders. I'm assuming you didn't have a previous (now razed) city which had access to that stretch of water.

 

[TractorBoy]

There are a couple of Roman tiles where that coastal patch is the closest so it's not implausible that open borders ended and you popped there. Check your global history to see if you recently had open borders end. Maybe you didn't remember where your Trireme was, so it teleported.

Good thinking, I am sure you have solved the conundrum. I can imagine a future scenario where you could declare war on an opponent, closing the open boarders agreement and in the process isolating their navy. I have often seen the mongols with three triremes in close proximity, the mind boggles!