Your question pertains to Part I of this incredible poem. The ship, although it left with good weather, has been subject to a tremendous storm that has blown it off course towards the South Pole. Now, the ship is surrounded by icebergs that are mast high and "green as emerald." As the crew try to see through the fog, all they can observe is more and more ice:

The ice was here, the ice was there,

The ice was all around:

It cracked and growled, and roared and howled,

Like noises in a swound!

Note the onomatopoeia in the description of how the ice sounded. We can only imagine how the fog, combined with the unnatural appearance of these massive icebergs and the sound they were making, struck the sailors with horror to their very beings. However, in the midst of this darkness and despair, a symbol of hope emerges:

At length did cross an Albatross,

Through the fog it came;

As if it had been a Christian soul,

We hailed it in God's name.

Thus the living thing that the sailors are able to see through the fog is the albatross, that is described in distinctly religious terms and is seen by the sailors as a symbol of some divine, righteous power. This of course makes the Mariner's crime in killing it all the more terrible.