Submarine volcanoes:

Submarine volcanoes are general features on the ocean floor. Some are energetic and, in shallow water, disclose their presence by blasting steam and rocky fragments high above the surface of the sea. Many others lie at such great rock bottom that the tremendous weight of the water above them prevents the explosive discharge of steam and gases, although they can be detected by hydrophones and staining of water because of volcanic gases. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather sharp pillars over their volcanic vents as compared to above-surface volcanos. In due time, they may break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes.