TUNDRA is one of several systems for expressing text-mode graphics using text characters and control characters setting positions and colors. ANSI Art was the main system of this sort, but a few others were invented during the BBS era.
The ansilove (and Ansilove-C and ansilove.js) open-source software can display TUNDRA among other formats.
TUNDRA files start with a byte of value 24 (decimal), followed by the ASCII string
More details of the format can be seen in the Ansilove source code (e.g., the function tnd in ansilove.js).
After the header, the file consists of a series of literal text characters (any that are not one of the few recognized control characters or the numeric parameters following them) and control sequences.
Most numbers are 32-bit unsigned big-endian integers. Color palette values are 24-bit RGB arrays.
Characters 1, 2, 4, and 6 (corresponding to Ctrl-A, Ctrl-B, Ctrl-D, and Ctrl-F) are the recognized control codes.
Character 1 is followed by two integers (32-bit) setting the new x and y cursor positions. 32 bits are rather overkill since these are setting positions on an 80 x 25 screen.
Character 2 is followed by a 24-bit RGB array to set the foreground color.
Character 4 is followed by a 24-bit RGB array to set the background color.
Character 6 is followed by two 24-bit RGB arrays to set both the foreground and background color.