Underground Features

Hello, I am new to worldgen datapacks and had a question reguarding underground features. How could I make a feature spawn only underground. It is not linked to any underground biome, I just want it to spawn in various biomes but only underground. As of right now, I just have it check every block from where it spawns to 128 blocks up but is poor on performance. If there is a better way to go about this, the information would be much appreciated.