The snark is a 90° stable glider reflector discovered by Mike Playle in April 2013. It is made up of two eaters, a block and a 31-bit still life, the heart of the Snark. It is currently the fastest and the smallest 90° stable glider reflector, both in terms of the population and the bounding box. Another commonly-used stabilization of the catalyst is 34 bits, and many other variants are available.
#C four Snark catalyst variants
#C Top: original variant by Mike Playle
#C Left: Shannon Omick (better clearance on a diagonal)
#C Right: Heinrich Koenig (better clearance on a different diagonal)
#C Bottom: Simon Ekström (better clearance on two diagonals)
x = 51, y = 52, rule = B3/S23
#C [[ THEME 6 GRID GRIDMAJOR 0 SUPPRESS THUMBLAUNCH ]]
#C [[ THEME 6 GRID GRIDMAJOR 0 ZOOM 8 HEIGHT 480 THUMBLAUNCH THUMBSIZE 2 ]]
four Snark catalyst variants --
Top: original variant by Mike Playle
Left: Shannon Omick (better clearance on a diagonal)
Right: Heinrich Koenig (better clearance on a different diagonal)
Bottom: Simon Ekström (better clearance on two diagonals)
The base reaction was discovered by Dietrich Leithner about 1998, but it consumed another block. A catalyst that could replace the block was found with Bellman, a program for searching catalytic reactions developed by Mike Playle.
Given its small repeat time, the snark made oscillators of previously unknown periods of 43 and 53 trivial.
It also made most large symmetrical Herschel-loop guns obsolete, since it is now possible to make use of the Herschel gliders with a shorter path of the Herschel track itself. 
The period 4 snark with repeat time 52 was used for p52+4n glider streams before the snark was discovered.