The geography of the Congo is kinda strange.
Okay, the geography of most African countries is strange. As someone or other said, Europe divided the region in a manner both completely self-serving and unbelievably haphazard, like monkeys at a salad bar. But Congo -- as usual -- is an extreme case. It might make sense to have a country that consisted of the Congo River basin. But Congo consists of about 3/4 of the Congo basin, plus a bunch of other odds and ends tacked on around the edges. Much of the north bank of the river is owned by Congo-Brazzaville (which is another country entirely). The northern and eastern fringes of the country aren't part of the Congo Basin --- they're the Great Lakes region, part of east rather than central Africa. And there's that weird long bulge into Zambia, which deserves a post of its own.
One of the obvious odd bits is at the mouth of the river. If you look closely at a map of the Congo, you'll notice that although it's immense -- "as big as the United States east of the Mississippi" is the example that's always used -- it just barely kisses the Atlantic Ocean: Congo has only only about 25 miles (40 km) of coastline.
Look even more closely, and you'll notice that the Congo coastline snips off a piece of Angola. Angola is a large country, and most of it is to the south of the Congo, but a little piece about the size of Rhode Island (or, if you're European, Montenegro) is north of the Congo's Atlantic coast. The reason for this is complicated, and involves a little history.
See, the lower stretches of the Congo River are pretty much impassible. Most of the river basin sits on a vast internal plateau a few hundred meters above sea level. The edge of the plateau is just 100 km or so inland from the coast. And the river goes over that edge in a whole series of immense rapids. "Immense" is really the operative word here, because the Congo is one of the world's great rivers, comparable to the Mississippi, Amazon or Nile. Nowhere else in the world does a river that big drop that fast. To give an American comparison, imagine if the Mississippi dropped a thousand feet between Baton Rouge and New Orleans.
So, early colonizers pretty much ignored the mouth of the Congo; it was navigable only for a day's travel inland, and its lower reaches were swampy and fever-haunted. The Portuguese dotted trading posts all along the coastline, but didn't much bother with the Congo. The French started moving in a bit further north, but also ignored the river.
But then King Leopold of Belgium -- determined to access that huge interior plateau and the vast river system that drained it -- built a chain of forts from the mouth of the river inland, connected them with a road up to the plateau, and started building a railroad. So when it came time to draw borders in Africa, Leopold claimed the lower reaches of the Congo River. This cut off a couple of small Portuguese settlements to the north, but nobody really cared much about upsetting the Portuguese.
Today that northern enclave is called Cabinda, and it has caused all sorts of trouble. To make a long story short, the Portuguese never could quite decide whether Angola (the big colony south of Congo) and Cabinda (the little bit north of it) were one colony or two. So when Angola declared independence from Portugal, Cabinda tried to declare independence from Angola. The resulting conflict lasted almost thirty years, and has only recently more or less died down.
Meanwhile, Congo still doesn't have a real seaport. Container ships come up the river to Mabadi, at the bottom of the rapids. The freight is then unloaded, put on trucks or the railroad (the rail line is still functioning, though just barely) and moved overland to Kinshasa. Geography has played a cruel joke here: Congo has a river system nearly as big as the Mississippi, but it doesn't connect to anything.