The Omega Tau podcast has a terrific episode on the logistics of shipping containers and why they're so cheap. Beyond just the obvious (floating heavy things is efficient), the shipping companies have ships with optimized hulls, special fuels, minute-by-minute scheduling, and hub setups. It's like the airline industry but with boats.
These things are the modern equivalent of ancient Greek amphora. The only difference Is that I don't think our descendants will be as excited when they run across these boxes due to the fact we're going to leave them with far more junk than the ancient Greeks left us.
The last sentence of the article linked below mentions the contents of the container. Ironically, it may or may not contain harmful contents, depending upon whether you believe NPR or BoingBoing.
Declares NPR: "And containers carry everything, from toxic chemicals to ribbon. The Monterey Bay container appears to be safe, though — according to the shipping company, it's full of radial tires."
Boing Boing rebuts: "But some objects do a better job of functioning as healthy artificial reefs than others. The aforementioned pile of tires, for instance, was a popular idea in the 1970s. Later, though, it turned out that sunken tires were leaching plastic compounds called PCBs into the water. Those compounds can cause cancer in animals, so the tire reefs, while attracting wildlife, were also, probably, killing some of that life over time."
It took some doing, but I finally found an acceptable number for the amount of containers shipped worldwide in a year. That happens to be 99.8 million (for just the top 20 exporters). So if the number lost is 10,000, then the percentage lost is 0.01% (barely a blip at all).