A short equipment list!
Lighting - preferably a head torch and a hand torch; plus a spare of some sort and enough batteries for each to last about 12 hours (although we should be no more than 5-8 if we go all the way into the mountain)
Sturdy clothing and footwear: pretty much as you would for a day on the hills, but old stuff that you don't mind getting dirty. Waterproofness is not a priority, but I find a cheap pair of waterproof trousers helps keep the dirt out (and I have a habit of sitting down in wet places). Wellies are adequate, with a good sole as there are a couple of slippery bits. And GLOVES! (gardening gloves or similar)(Thank you Walrus!)
A helmet is preferable, in case you bump your head on the ceiling.
Food and drink - again, allow for several hours: a mix of chocolate (fast energy replenishment) and sarnies/pies/pasties (slow burn) plus drink.
(by now you'll have twigged that a rucksack of sorts is a good idea!)
Cameras! If you want pics that come out decently, a tripod is an absolute must (mine is the cheapest cheap that Tesco do, a Hama Star 42). There will be plenty of opportunities for taking photos :)
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Right then, what have I forgotten? 😉
Hello again darkness, my old friend...