That’s what the world wants to know as the footage racks up massive amounts of views on YouTube.
The footage was uploaded to YouTube on April 15 with the title “Monster in Lough Foyle!” and has received close to one million views so far.
In the description, filmmaker Conall Melarkey says that whales have been spottted in Lough Foyle before, so there could very well be a reasonable explanation.
What do YOU think it was? Check it out below.