Super Mario Bros. Plus (SMario+ for short) is an insanely hard hack created by U1 in 2002, taking all levels from the original SMB, and putting huge amounts of difficulty on them. This is said to be COMPLETELY impossible to complete without the usage of save-states, slowdowns, etc.
A secret level is apparently available known as World 52-1 (What I'd call in the hack The Shaded World). My run takes advantage of that, completing the hack completely in just over 1 min. 15 sec.! Astonishing!
You can download the IPS patch for this hack here: https://drive.google.com/open?id=1MMiSdvY-GlXegm4OY9qAe0hBWXUUca9c
There is also a run of its easier, but still insanely hard sequel, Super Mario Bros. Minus (SMario- for short), if you're looking for more hard level design action here.
An “8-4%” TAS of this hack can also be seen here.