For as long as the series has existed, there have been few things as satisfying in Civilization as building a city on a single tile/hex of Earth separating two larger landmasses. They're known as Canal Cities. In their best examples, you'll see them called Canal Porn.
If you play Civ a lot you probably already know what this means, but if not, let me explain: Building a city on a canal in Civ is the best. It feels like settling into your favourite pair of slippers and getting the best seat in the movie theatre at the same time.
Here's an example from Civ V:
That city in the middle, Damascus? That's a Canal City, in a good enough spot that it qualifies as Canal Porn. They haven't always been the optimal place to build a city in terms of production and yield (different Civ games have offered differing levels of value on coast and ocean tiles), but it's always the most strategically satisfying.
They lock down, Walder Frey-style, the land passage between the territories on either side of it. They can control the sea lanes separating that land. Best and most importantly of all, they allow your naval units (and those of your friends) to travel instantly between those seas either side of them, without the need to sail all the way around a continent.
Hence the name Canal City; they're named after the Panama Canal.
A good Canal City being built is planting a flag down in the ground that says "YOU SHALL NOT PASS", while at the same time allowing you the Civilization equivalent of a free waterslide.
A long-standing obsession amongst the Civ reddit community, where countless examples have been posted and compared over the years, Canal Cities tend to not just be important, but look cool as well. Especially when you can string a few together and do something like this:
It's like a battleship teleportation system.
So why the obsession? Aside from the obvious strategic value in them, I've always figured it's as much a result of Civ's map generation - making an ideal location for one relatively scarce - as it is any human thought that goes into planning them. A combination of luck and strategy that creates something you want to share.
A good Canal City is also such a movement/time saver that you always get a little kick from using them to send your navies through. It feels like a cheat. Something mildly illicit. As though you've found an underhanded way to get around glacial ocean movement speeds - one of Civ's more tedious design decisions - and want to share in this tiniest and most inconsequential of thrills with others and compare notes. See whose canal was the canaliest.
A Canal City is essentially this. Only with little battleships and ironclads passing through.
Now that Civ VI is out, of course, folks are free to explore how canals are shaping up in the new game, and share more contemporary examples of the phenomenon. Like this one below, which is incredible; it's not like canals aren't defensively-oriented enough, but this one gets cliffs and mountains as well (not to mention a bounty of sea resources).
As someone whose games thus far have lumped me in deserts, icy wastelands and endless grassy plains, I'm very jealous.
Also, apologies to anyone who read "Canal Porn" expecting to see Civ players actually fucking a piece of the map, but like... come on.