Early Monday morning, an unknown hum reverberated across Costa Rica, leaving citizens and experts throughly confused. According to OVSICORI, the Costa Rica Volcanologist and Seismologist Organization, there was no tectonic activity at the time. What were some other suspects for this brontide, or aural cryptid? Explains The Tico Times:

Volcanologists discounted a volcanic eruption; nor was it a supersonic aircraft, because the powerful radar at Juan Santamaría International Airport outside of San José picked up no planes at the time, not even subsonic ones.

Also, there were no climatological phenomena or strange weather conditions. By nightfall Monday, the speculation was still going along briskly with the major vote going to fireworks at the festival at Zapote, in San José. But many said it sounded like no fireworks they had ever heard. Mario Sánchez, spokesman for the National Meteorological Institute, discounted a storm because the only thing that would sound like the noise described would be a storm accompanied by lightning – and early Monday morning was clear with a full moon.

The search is still on for the source of this phantom grumble, you can listen to this Cthulhu flatulence above. Hat tip to Akinaspeedstar.