Sentence:  Prior to going back out to Mystery Cave again we spent a lot of time preparing. We made a <WSD>squeeze box</WSD>, which is a wooden box the opening of which can be adjusted in size. We could then crawl through the opening and measure to see how tight of a squeeze we could fit through. 
Meaning of the word marked with <WSD> tags:  A box with an adjustable opening used by cavers to practise crawling through tight spaces.