Other factors such as iron-rich dust influx from large desert areas such as the Sahara are thought to play a role in causing HABs. Some algal blooms on the Pacific coast have also been linked to natural occurrences of large-scale climatic oscillations such as El Niño events. HABs are also linked to heavy rainfall. Although HABs in the Gulf of Mexico were witnessed in the early 1500s by explorer Cabeza de Vaca, it is unclear what initiates these blooms and how large a role nanthropogenic and natural factors play in their development.