Underwater volcano found near Italy

A huge underwater volcano has been discovered 40km (25 miles) off the southern coast of Sicily.
The Italian scientists who discovered the volcano have named it Empedocles.
The volcano's base covers an area larger than Rome, and it's higher than Paris' Eiffel Tower with one peak only seven metres below the sea's surface.
Empedocles is dormant and luckily shows no sign of imminent eruption. Mount Etna, Europe's largest active volcano, lies 100 km (62 miles) to its north.
The base of the new underwater volcano measures 750 square km (290 square miles) and it sticks out of the ocean floor by some 400 m (2460 ft).

If it's that big - how come they didn't notice it before?