Ecco the Dolphin/Dark Water

From the start, swim right, and swim downward until you reach solid land. There is a moving shell just below the first prehistoric shark, which will need to be avoided. When you see the solid land, turn right and head up. You will find a glyph - sing to it to become invincible.

With your invincibility, head down. There are two moving shells, and they can still crush you even if you have invincibility. Follow the tunnel until you see the key glyph. Touch it, and sing to it to become invincible once again.

Under normal play, you will have to complete the next section quickly, due to lack of air resupply. Backtrack until you find the barrier glyph in the downward shaft. Sing to it with the key, and it will fall away. Swim down the second shaft, and make a left turn. Follow the tunnel until you see a seahorse.

This seahorse can be defeated by hitting it eight times, but it's easier to slip behind it as it shifts back and forth. The tunnel behind it is filled with shifting shells that can easily crush you. At the end of that tunnel is a key glyph that lets you reach the asterite, as well as provides invincibility.

Return to the first barrier glyph, head left, and sing to the second glyph. Swim, and look for a hidden passage at the top of the short tunnel.

The asterite can damage you by contact, as well as shoot an electric charge if you hold still for too long. To damage it safely, wait until one of the globes is nearing the outward part of the rotation, and charge the ball. It will begin to flicker. You will need to repeat this three more times on a globe of the same color.

Once the asterite falls, you will warp back to the present in Deep Water. The set of levels plays almost identically to what you've encountered before, the next new level is ../The Tube/.