The dome is very high, and sheds down light when the gleam of the sea strikes inward.

From the gloomy mouth of it, as far as they could venture, the lapping of the wavelets could be heard all round it, without a boat, or even a balk of wood to break it.

Then they tried echo, whose clear answer hesitates where any soft material is; but the shout rang only of hard rock and glassy water.

To make assurance doubly sure, they lit a blue-light, and sent it floating through the depths, while they held their position with two boat-hooks and a fender.

The cavern was lit up with a very fine effect, but not a soul inside of it to animate the scene.