The phrase "loomed up" implies a sudden appearance or emergence of something, typically large, in one's view. Some common antonyms for "loomed up" could include words such as "disappeared", "receded", "vanished", "faded", "dissolved", or "retreated". These words convey a sense of something gradually fading away or becoming less visible, rather than a sudden and looming presence. Similarly, synonyms for "loomed up" could include "appeared suddenly", "rose up", "towered over", or "emerged from the shadows". By contrast, the antonyms for "loomed up" suggest a more gradual and less intimidating process of disappearance or retreat.