Sentence:  through the sash window I could see the black truck pulling up the drive towards the main road, the silver caravan coming behind like a <WSD>drogue</WSD> that was preventing the gypsies from submerging, escaping into the very centre of the earth. 
Meaning of the word marked with <WSD> tags:  A floating object attached to the end of a harpoon line to slow a whale down and prevent it from diving.