Where is the infinite (99) lives staircase in the original Super Mario Bros?

I know there's a staircase at the end of one of the levels where a well-timed jump onto a turtle coming down a staircase will allow me to keep landing on the same shell until I max out my extra lives.

Where is it?


Solution 1:

It appears to be at the end of stage 3-1. (source)

I've unfortunately never been able to perform this trick. The potential of 99 lives does you no good if you die every time you go for it.

Solution 2:

It is the level 3-1.

Solution 3:

There is another one in Stage 7-1 with a Buzzy Beetle at the end staircase.