For the second time in three years, the 1972 Miami Dolphins are getting nervous about sharing their place in history.

The Dolphins, the only NFL team ever to go undefeated, now see two teams — The Indianapolis Colts and the New Orleans Saints — threatening to accomplish what they did nearly four decades ago.

Two years ago, the Dolphins watched and waited as the New England Patriots came within one game of joining them — only to lose in the Super Bowl.

What makes it so difficult to go undefeated?

First, of course, you have be a skilled, tough team.

Second, there is so much competition — every team you play is professional — that it’s always possible to face someone who can beat you.

Third, there is the mental challenge. It’s so hard to get up emotionally for every game. And the more you win, the harder it is to take the so-called weaker teams seriously. Finally, there’s always a danger that the streak can become a distraction — winning is so difficult that the moment you stop preparing and start thinking about the record books, you’ve lost a crucial bit of focus. It’s like the story of the tortoise and the hare. Even though the hare ran faster, it lost the race to the tortoise because it stopped running and started to think.

Two weeks ago the Saints saw how hard it is to stay undefeated. Playing the lowly Redskins, they needed some late heroics — as well as some Redskins mistakes — to keep their streak alive.

By the way, there’s a legend that the surviving members of the 1972 Dolphins still get together every year for a toast when the last undefeated team loses. Apparently it’s nothing more than a legend.

But toast or not, the Dolphins have every right to be proud what they accomplished — it is so difficult to do.