As some of the oil and gas leases in the Gulf of Mexico and elsewhere play out, drilling companies are left with the problem, what to do with those multimillion dollar oil platforms?

Federal lease agreements require oil and gas companies drilling in U.S. waters to leave the ocean floor as they found it. But tearing the platforms down, hauling them to shore and selling them for scrap can cost tens of millions of dollars. So the industry is studying alternative arrangements.

• From the day they are put in place, the structures act like artificial reefs -- first attracting coral and sponges that attach to them, then shrimp and crab follow, along with fish, all congregating to find shelter and things to eat.

• So the leading alternative to scraping the structures -- which can tower nearly as tall as the Empire State Building -- is to leave them as artificial reefs, which the federal government sometimes permits.

• The process of "reefing" consists of removing all of the above-water structure, then cutting the below-water structure at several points and placing those next to the remaining anchored structure on the bottom.

• Other alternatives have been suggested -- including turning the platforms into prisons or creating marine resorts.

The Gulf is home to 3,687 oil and gas platforms, but reserves in the shallower waters are dwindling. So experts expect the number of platforms along the shelf to decline 30 percent over the next 25 years. Companies are expected to install 142 platforms in the area each year through 2023 -- but they are also expected to decommission 186 a year.

So far, more than 160 platforms in the Gulf have been turned into reefs.

Source: Thadeus Herrick, "The Afterlife of Oil Rigs," Wall Street Journal, June 27, 2001.