The inaccessible section at the south end of the Underground Path (N-S) map.
Underground Path's inaccessible section is an inaccessible part of the Underground Path between Routes 5 and 6 (i.e. the North-South one) in Pokémon Red, Green, Blue, and Yellow. Despite the similarity to some unused contents, the disassembly suggests that it is actually caused by mismatched map size and map data. However, the mismatch itself is likely to be a development leftover.
The section is located beyond the south end of the normal Underground Path. It is normally inaccessible and invisible, since even standing on the southmost normally walkable tile will not scroll it onto the screen. However, it can be easily accessed by walking through walls. If the player tries to move further to the south or the east, the game will crash due to a freeze tile (i.e. invalid map connection to map 0xFF).
The section exists because the size of the Underground Path (N-S) is defined to be 24 * 4 blocks (i.e. 48 * 8 tiles), yet the corresponding map data only have 23 * 4 = 92 blocks. When forced to load the last row of blocks, the game goes into the piece of data in ROM after the N-S Underground Path map data, which happens to be the map data for the E-W Underground Path. Since both Underground Paths have the same tileset, the inaccessible section looks exactly like the top left corner of the E-W Underground Path.
It has been theorized that an early version of the N-S Underground Path did have 24 * 4 blocks, with a staircase in the middle connecting to another Underground Path (not in the final game). Later, the developers removed the row of blocks containing the staircase, but forgot to change the map size.