What is a common reason flames can ignite underwater?

Flames can ignite underwater primarily due to the presence of combustible materials such as oils combined with salts. These substances can float on water and create an environment where ignition is possible despite the surrounding water, which typically acts as a suppressant for fires. When oil is present, especially in significant quantities, it can form a surface layer that is not only flammable but can also be ignited by heat or spark, thus allowing flames to propagate in a liquid environment where normal combustion is unlikely.

Factors like high water pressure might influence the behavior of fire or combustion in terms of how gases are released, but they do not directly cause flames to ignite. Temperature fluctuations can influence the state of substances and their readiness to combust, but they are not a direct cause of underwater ignition. Reactions with marine life might introduce potential sources of ignition in very specific scenarios, but they are not commonly recognized as a significant reason for flames igniting underwater. Thus, the presence of oil and salt is the most relevant and common cause for underwater fires.