This classic puzzle popped up on Quora.
In case you haven't seen this before and want to have a think I have left a gap below, so you need to scroll down to reach the main business of the post, which would give the game away if you read it!
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Invertible words are words made entirely out of invertible letters:
B, C, D, E, H, I, K, X
which look exactly the same if you turn them upside-down. (We have to use capital letters, it obviously doesn't work with lowercase.) Seeing the car park puzzle made me wonder what was the longest invertible word I could find. So I found a public domain word list of about 170,000 words and wrote a short Python program to search it for invertible words.
They are quite rare: I only found about 400, so that about 0.24%. The longest dictionary words were all 8 letters
BEDECKED
BOOHOOED
CHECKBOX
COOKBOOK
EXCEEDED
HOODOOED
However the word list seems to include a few random place names—perhaps they are ones the compiler of the list had a special fondness for—and so my program also found the 10-letter OKEECHOBEE.
Now this is a town in Florida. It has a lake of about 2.000 km2, serious lake, and about 5,000 citizens. And it has a city limit sign as you approach: here it is upside-down
Unfortunately we have lost the last two letters but you can see the name is indeed the same upside-down. AI Overview has this to say about the name, which is also rather interesting, since it evidently refers to the lake
Hitchiti Indian words oki (water) and chubi (big), translating to "big water".