ICETOP is a partially implemented stage in Super Smash Bros. Melee that can be seen in the game's debug mode by Action Replay, but only accessed by other hacks. The stage was intended to be a second Ice Climber stage beyond Icicle Mountain; Ice Climber is the only universe with a starter character in Melee to have only one stage. The stage is also known as 10-2, an identically-functioning entry in the debug menu reserved for an Adventure Mode stage following Icicle Mountain (which is stage 10-1).
The game does not load the stage properly if selected through the debug menu, but instead requires other hacks to load. Doing so only yields Icicle Mountain with no music.
According to an answer given by Masahiro Sakurai on the Melee website's Q&A section, a second Ice Climber stage was indeed planned but ultimately scrapped, with the intended name 山頂 (Sanchō), meaning "Peak", "Mountaintop", or "Summit". This seemingly makes it a conceptual predecessor to the Ice Climber stage in Super Smash Bros. Brawl, Summit, although the Japanese name for that stage is instead 頂上 (Chōjō), another possible word for "Summit", though the files of Ultimate do give it the name of "Ice_Top".