The ship's all wrong.

I'm very much afraid some one has been opening a book and this river has got out.' Philip hurried out on deck, and by the light of the lamps from the cabin, gazed out at the banks of the river.

At least he looked for them.
But there weren't any banks.

Instead, steep and rugged cliffs rose on each side, and overhead, instead of a starry sky, was a great arched roof of a cavern glistening with moisture and dark as a raven's feathers.
'We must turn back,' said Philip.

'I don't like this at all.' 'Unfortunately,' said the parrot, 'there is no room to turn back, and the _Lightning Loose_ is not constructed for going backwards.' 'Oh, dear,' whispered Brenda, 'I wish we hadn't come.