Crystal wall is a mile wide

Soon they neared the source of that light, but to their surprise they could not proceed.  The light came from beyond a semi-translucent wall of crystal.  Behind that wall, brilliant lights glowed and flickered.  At the base of the wall, the underground river flowed into a vast, deep lake.  The black waters shimmered beneath their hull and lapped against the crystal barrier.
"I don't think we can go any further," Captain Dolan announced.  But even as he spoke, a tiny hole appeared in the center of the crystalline wall.  Michael spotted it first and pointed it out to the others.
As the rest of the crew watched in amazement, the hole continued to expand, like a huge crystal iris.  They watched hopefully as it opened wider and wider, until a gateway a mile in diameter yawned before the airship's nose.