As Ralph had said, it was not so dark as might have been expected.

In fact, about twenty feet forward there was a dim light on the right-hand wall.
The captain, still followed by Ralph, now moved on until they came to this lighted place, and found it was an open doorway.

Both heads together, they peeped in, and saw it was an opening like a doorway into a chamber about fifteen feet square and with very high walls.

They scarcely needed the lantern to examine it, for a jagged opening in the roof let in a good deal of light.
Passing into this chamber, keeping a good watch out for pitfalls as he moved on, and forgetting, in his excitement, that he might go so far that he could not hear Maka, should he call, the captain saw to the right another open doorway, on the other side of which was another chamber, about the size of the one they had first entered.

One side of this was a good deal broken away, and through a fracture three or four feet wide the light entered freely, as if from the open air.