How did early explorers determine the depth of and ocean

What statistics are to determine the development of a nation?

Witch of the following volcano hazards is made up of rocky particles about the size of a grain of sand

Why is the huronian supergroup rock formation particularly interesting to scientists? because it looks like a cross-section of a giant, stone encyclopedia because it formed during the period when oxygen began to accumulate in the atmosphere because it contains unusually large amounts of oxygen and sulfur because it dramatically altered the planet's development when it first formed