The 1989 sequel to Tetris that extends the pit into three dimensions is Welltris.
Welltris was designed by Tetris creator Alexey Pajitnov with Andrei Sgenov and developed by Doka. Unlike ordinary Tetris, it presents an 8×8 floor surrounded by four walls, viewed from above. The falling pieces remain two-dimensional, but they descend along the surfaces of a three-dimensional well.
Players move and rotate shapes around the walls, where pieces slide until they meet an edge or another block. Completed horizontal or vertical lines disappear, while a wall can become temporarily frozen if a piece rests against it. Freezing all four walls ends the game.
The title is sometimes confused with Blockout, another 1989 puzzle game that inspired Welltris, or with later games marketed as “3D Tetris.” Welltris is the specific official Tetris-series sequel described by the question. It first appeared on MS-DOS, followed by ports to systems including the Macintosh, Amiga, and Atari ST.