MADRID -- Local media reported on Wednesday that there were signs that an underwater volcano has started to erupt off the southwest coast of El Hierro island. El Hierro is located off the west coast of Africa and is the most westerly part of the Canary Islands. It has witnessed an increase in seismic activity over the past days with 94 quakes recorded in the early hours of Tuesday. Authorities said the quakes registered between magnitude 2.6 and 4.2 on the Richter scale. Seven were felt by residents on the island, which is home to about 10,000. On Wednesday morning, 40 more tremors were recorded. Experts are now evaluating aerial photos of bubbles on the surface of the sea as well as the appearance of a white stain. They are trying to pinpoint the origin of the bubbles, which may have been produced as a result of sulfur being spewed from an underwater vent.