The one-person submarine is great for traveling fast underwater. Even together, because a freediver can easily hold onto it on the outside. However despite it's compactness, it is still not small enough for tighter caves and that's exactly where Blaze and May have to go.
They reach such a tight spot just in time for Amy's air to run out. With slightly red-purple cheeks from the strain, the hedgehog girl takes Blaze's spot who just took a last breath from the air chamber of their vehicle.
Blaze checks on Amy that she is okey before she heads on through the tunnel on her own, leaving Amy to recover.

The water is cold and it is surely quite the situation to have a ceiling above your head underwater all the time. But Blaze would not be Blaze if she would be scared of just some danger.
Bravely she kicks through the increasingly tight tunnels until she begins to feel the tightness in her chest, reminding her that she would need air soon. She is on the verge to return when a sparkle catches her glimpse just past the next corner. The crystal!
A smile crosses her face and she ignores the first spasm of her chest when she dashes for the crystal.
To her misfortune, this particular cave is much less stable than it seems. The currents caused by her kicks are enough to trigger a chain reaction that causes debris of rock to drop from the ceiling. Due to the water, they don't fall as fast to injure Blaze, but the collision draws a cloud of precious bubbles from her and the cat girl quickly ends up pinned, unable to move!
The crystal she soughed after remains just out of reach...

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This is a comic that was on my mind for literally ages.
Thank you https://www.deviantart.com/uwdennis for realizing it entirely! ^w^