A word for “cave, mine, underground dwelling” Tolkien considered in Notes on Names (NN) from 1957, derived from the root √PHELEK as a possible replacement for felya “mine, boring, tunnel, underground dwel[ling]” < ✶phelgā (PE17/118).
Neo-Quenya: I prefer the root form √PHELEG over √PHELEK (see that entry for details) and as a result prefer felya over felco.
felco noun "cave, mine, underground dwelling" (PE17:118); also felca, felehta