Retired Olympic superstar Michael Phelps is said to have gone back into training. (Toni L. Sandys/The Washington Post)

Big swimming news: Michael Phelps is back in the pool.

Reports indicate that Phelps, probably the greatest swimmer ever, is training with his longtime coach, Bob Bowman, at the North Baltimore Aquatic Club.

Phelps retired after the 2012 Summer Olympics, saying he had accomplished everything he had wanted to in swimming. Phelps won a record 22 Olympic medals, including 18 gold medals, in three Olympic Games. He won an incredible eight gold medals in the 2008 Olympics.

Athletes make comebacks all the time. Some make it back to the top, while other comebacks flop. Let’s take a look at some.

Before Phelps, Mark Spitz was considered the greatest Olympic swimmer ever. Spitz won seven gold medals at the 1972 Summer Games in Munich. Twenty years later, at age 41, he tried a comeback. He found that he could not keep up with younger swimmers. Spitz failed to qualify for the U.S. Olympic team.

Swimmer Dara Torres took time off after the 2000 Olympics and had a baby. She had already won nine Olympic medals at the 1984, 1988, 1992 and 2000 Games.

Torres came back for the 2008 Olympics at age 41. That’s old for a competitive swimmer, but she added three silver medals to her Olympic collection. Torres tried to make one more Olympic team, in 2012. But she missed qualifying by a split second.

Another athlete who took time off to start a family was tennis champion Kim Clijsters from Belgium. She retired in 2007 and gave birth to a baby girl in 2008.

In 2009, Clijsters decided to play tennis again and was better than ever. She won three major championships — one Australian Open and two U.S. Opens — after her return to the sport. She had won only one major championship before her retirement.

Clijsters retired again in 2012 and had a second child in September of this year. But who knows? Maybe she will try another comeback.

So can Michael Phelps come back and be an Olympic champion again? Phelps is only 28 years old, so he’s not that old for an Olympic swimmer. He has time to work his way into world-class shape.

The next swimming world championships will take place in the summer of 2015 in Russia. The next Summer Olympics will be held in Rio de Janeiro, Brazil, in 2016. Phelps will be 31 years old then.

Swimming at the international level is super competitive. Phelps will have to be ready to be the best in the world. Olympic medals are often decided by fractions of seconds.

And comebacks can be tricky.

Fred Bowen is the author of 19 sports books for kids. He has written books about baseball, basketball, football and soccer — but none about swimming.