On the night of April 20, 2010, the Deepwater Horizon, a rig owned by Transocean Ltd., burst into flames after drilling a well for BP PLC, killing 11 workers on or near the drilling floor.
The rest of the crew evacuated, but two days later the rig toppled into the Gulf and sank to the sea floor. The bodies were never recovered.
Over the next 85 days, more than 200 million gallons of crude oil spilled into the sea.
Some relatives of the men who died are flying over the Gulf of Mexico to mark the one-year anniversary. They were expected to circle the site a few times in a helicopter.
In a statement, President Barack Obama paid tribute to the 11 men killed in the explosion and thanked the thousands of people who responded to the disaster.
Meanwhile, accumulated oil is believed to lie on the bottom of the Gulf, and it still shows up as a thick, gooey black crust along miles of Louisiana's shoreline.
The Associated Press contributed to this report.