When Nintendo decided to re-release the primary Super Mario back catalog for the Game Boy Advance, the company decided to include small amounts of new content. The remake of Yoshi's Island, Super Mario Advance 3, was the first in the series to include actual new levels. Six of 'em, to be exact. Then Super Mario Advance 4 (the remake of Super Mario Bros. 3) came along and Nintendo created over two dozen new levels to insert into the game (note that these are not the lost levels of the original Super Mario Bros. 3).
However, instead of loading them into the game pak the company decided to release them in dribs and drabs for the soon-to-be failed e-Reader accessory. The e-Reader fizzled before Nintendo could release all of the levels, with less than a third of the developed content making it into the hands of fans. Now a group of game enthusiast hackers are working to rediscover and release these latest lost levels, restoring yet another piece of Nintendo history that has been swept under the rug.
For those not familiar with the method in which players could load new levels into Super Mario Advance 4, here's a basic rundown. In order to make the whole thing work, players each need two Game Boy Advance units, one copy of Super Mario Advance 4, one e-Reader, and the appropriate cable to connect it all together. The game goes into one Game Boy Advance while the e-Reader goes into the other. Then players had to buy packs of what are essentially playing cards with small dot codes printed on each card. By scanning those dot coded cards into the e-Reader, new levels encoded by those dot codes are made available to the game. Few people had access to all of that pricey equipment, meaning that most owners of Super Mario Advance 4 never had the chance to play the newly designed levels.
The group working to release these lost levels (they seemingly go by no name publicly from what I can find) initially began to compile information on just what these lost levels were all about. Levels 01-05 are Classic and Promo levels in which the familiar World 1 and World 2-2 of the original Super Mario Bros. have been redone in Super Mario Bros. 3 clothes along with a new fortress level. The only new levels released in America are numbered 01-10 and include a new desert level in the tradition of World 8-2 in the original game, new autoscrolling sky levels, and a level that pays tribute to Super Mario Bros. 2 (Super Mario Advance) with pickable vegetables.
Levels 11-30 were released in Japan for the Japanese version of the game, but Japanese dot codes do not work with the American version. These are the bulk of the lost levels and include new airships, new mazes, elements from both Super Mario World and Yoshi's Island, plus a grand final showdown with Bowser himself. Rounding out the collection of material is a look at the truly lost levels; the levels shown at press events and trade shows but were never released to the public in any form.
Now for the bad news. The website that catalogs and releases all of this material also contains a download of the actual Super Mario Advance 4 ROM file for emulation which, as you'll recall, means that due to United States law I cannot link directly to the website. However, it is out there if you search long enough and read enough supplementary material. Please do not ask for the link, as I cannot provide it. With that said, enjoy this look at more lost Nintendo material and keep the hope alive that the company may use these levels in a future iteration of the game.