"Impossible" tiles"Impossible" tiles are those the community have decided that there is no realistic possibility of being visited. Examples of impossible tiles are military bases, munitions dumps, open-cast mines and private game reserves.
Impossible tiles are the tiles that make it impossible to grow a max square any further. In fact this was the original motivation for introducing the notion of the "max cluster". A tile is in your cluster is you have visited it and have visited all the tiles around it. Thus an impossible tile will never be in your cluster and nor will the 4 tiles that surround the impossible tile. So the "cost" of an impossible tile is that it reduces your max cluster by 5, but at least it doesn't stop it from growing completely.
There will always be differences of opinion on what makes a tile "impossible". However the balance of opinion of the community seems to be to keep the list of impossible tiles to a minimum. Very difficult tiles are not impossible tiles and in fact some of the most satisfying tiles to tick are those that are difficult to get to. So tiles that are in the middle of lake (which can potentially be kayaked to) or the top of a mountain (which might be climbed) are not impossible.
Reporting impossible tiles** Currently not available on phones **
If you know of any tiles that should be marked as impossible but are not, please tell us! To do this, open the heatmap and find the tile. If you are sufficiently zoomed in, clicking on the tile will show you a "Submit your own notes" form on the left. Describe why the tile is impossible and submit the form. At the current submissions will be reviewed manually before being shown in red on the heatmap in future.