New Super Mario Bros. underground background ripped by Demon2Warrior on The Spriters Resource and converted to SMW by MORC.
This background is compatible with the NSMB underground foreground by Rykon-V73 and Treeki.
This works in vertical levels with H-Scroll:variable and V-Scroll:variable.
ExGFX259 goes on BG1 and ExGFX25A goes on BG2.
Thanks to TheMorganah and the MarioHacks blog for the tutorial explaining how to convert the graphics.