“There must be more than one entrance to this city,” spoke the professor. “I think I’ll go and hunt for another. When we find it we can take the automobile with us and escape to-night. I wish to be the first person to announce this discovery to the world.”
“That’s the idea!” exclaimed Ned. “I’ll go along to help hunt for another passage, while Bob and Jerry can stay on guard.”
“In the meanwhile I’m going to have my swim,” said Bob. He went into the tank-room, and immediately uttered a cry.
“What’s the matter?” called Jerry.
“The water has all run out,” replied Bob, “and there’s a big hole here!”
The others came in on the run. They saw that the swimming-pool was empty. Only a little water remained on the bottom in small puddles. They also saw that the pool was made with an incline of stone leading from the floor level down to the bottom. In the side opposite from where the incline was a big black hole showed itself. When the water was at the normal level this hole was invisible. Once the water had lowered it was plain to see.
“What made the water go out?” asked Bob.
“Probably a gate at the end of the tunnel leading from the tank was opened,” replied the naturalist. “Or it may be an automatic arrangement, so that when the tank gets filled up to a certain height the water shuts itself off. So we’ll defer our bath until the water rises. Perhaps the tides may have some effect on it. We can only wait and see.”
“That tunnel is big enough to drive our auto through,” observed Bob.