But it was penetrable, then.
"We crossed.

As we were about to enter the cleft there issued out of it a most unusual and disconcerting chorus of sounds--wailings, crashings, splinterings." I started, shot a look at Dick; absorbed, he was drinking in Ventnor's every word.
"So unusual, so--well, disconcerting is the best word I can think of, that we were not encouraged to proceed.

Also the peculiar unpleasantness of the hollow was increasing rapidly.
"We made the best time we could back to the fortress.

And when next we tried to go through the hollow, to search for another outlet--we couldn't.

You know why," he ended abruptly.
"But men in ancient armor.