You know ExxonMobil (NYSE: XOM) is a humongous oil company. Maybe you even know about its "cleaner" efforts in support of electric cars and creating biofuels from algae.

But you probably don't know this: ExxonMobil, and other oil companies, have been spilling massive amounts of oil in Africa for some 50 years, with the world looking the other way. It's largely been a non-issue for the companies, but times may be changing. If they're called to account for their actions, it could be costly.

Let's look at BP (NYSE: BP) briefly, for some perspective. It's not known just how many gallons spilled into the Gulf over the past three months. I've seen estimates ranging from 11 million gallons to more than 140 million. It's currently estimated that the total cost of the disaster for BP will be between $17 billion and $60 billion. Big numbers, eh?

Now let's swing over to Africa; Nigeria, in particular, where much drilling has taken place for decades. A June New York Times article noted that the Niger Delta "has endured the equivalent of the Exxon Valdez spill every year for 50 years by some estimates."

We know that much of Africa is destitute; it can't help to have vast swaths of the environment polluted and rendered unproductive. Just a few months ago, a Royal Dutch Shell (NYSE: RDS-A) pipe reportedly burst, killing off an area rich in shrimp and crab. Before that, an ExxonMobil spill lasted several weeks, leaving fishermen with oily nets and no fish.

The estimated African total spillage is appalling: as much as 546 million gallons of oil over 50 years. (The Exxon Valdez disaster deposited about 11 million gallons.) Shell, ExxonMobil, and others often point to sabotage and corruption as contributing factors to the spills, but that's disputed by environmentalists and locals.

To their credit, lots of companies are looking into alternative energies these days. Chevron (NYSE: CVX) is another algae biofuel hopeful, for example, while Dow Chemical (NYSE: DOW) is investing in algae-based ethanol. But that doesn't change the fact that oil is spilling much too frequently and in vast amounts in Africa, doing terrible damage.

If the responsible companies are held more accountable for that at some point, it could prove quite damaging to their bottom lines. As investors, we should factor these liabilities into our calculations and decisions. As global citizens, we should support efforts to right these wrongs.

For a more uplifting perspective on investing, take a few moments to learn about a stock you shouldn't sell. It doesn't spill oil, either.