As Nepal makes 97 peaks free to climb, we look at the science of overtourism – especially on mountains. How do so many visitors affect these environments, and what can be done? Plus, are mountains carbon sources, or carbon sinks? And why do we feel the drive to summit these peaks anyway? A psychologist specialising in thrill-seeking explains.

Also on the show, we take your questions, read out your comments, and look at what it takes to physically measure the height of a peak like Everest.

All that, plus many more Unexpected Elements.

Presenter: Caroline Steel, with Chhavi Sachdev and Leonie Joubert Producers: Margaret Sessa Hawkins, with Alice Lipscombe-Southwell, Imaan Moin and Lucy Davies