This last is another long-standing mystery to me.

I just realized that you've answered a long-standing mystery.

The systematic position of the Silktail has been a long-standing mystery.

The position of the trogons within the class Aves has been a long-standing mystery.

The origin of so many globulars has been a long-standing mystery.

One long-standing mystery of the painting is why Mona Lisa has only very faint eyebrows and apparently does not have any eyelashes.

The constant ocean salinity was a long-standing mystery, because no process counterbalancing the salt influx from rivers was known.

The long-standing mystery of these exploding stars lies in precisely how they explode.

The systematic affinities of the genus have been a long-standing mystery.