I did not know that yesterday!

I did a little searching and found an answer of sorts on the web site of Crystal Geyser. In the FAQ section, someone asked why the company's bottle water has an expiration date. The answer explains that some states require all food and beverages, including bottled water, to state an expiration date on the label. The company says, "we feel that an average storage of two years will help ensure optimal quality." They go on to recommend that you store bottled water in a cool and dry
environment, away from non-food products.

I then turned to the MadSci Network for more information. Answering the question of why bottled water has an expiration date, the site states that over time, the water picks up flavors from its packaging. These flavors, combined with warm storage temperatures, contribute to the "generation of musty tastes."

According to the International Bottled Water Association, the FDA has not established a shelf life for water, so you can store those bottles indefinitely. As long as the packaging is intact and unopened, you shouldn't need to worry about any type of bacteria or microorganisms growing in the water. If the bottle has been opened, however, it can grow bacteria and algae if it is not consumed within 2 weeks.

So while it doesn't appear that unopened bottles of water actually go "bad" in the sense that they would make you sick, they might not taste the greatest after several years of sitting on the shelf collecting dust.