Trench warfare and caving... two cold, wet, and dangerous activities, which have both inspired creative ways to stay warm.
Caver and hypothermia expert Jonathan Palmer invented the Palmer Furnace, for use underground. Cavers, especially expedition cavers, often traverse tight, wet passages, in caves which may have an ambient temperature in the 30's or 40's Fahrenheit. This is commonly followed by long periods of inactivity, sitting on cold ground, surveying new passage. This combination can easily result in hypothermia.
Palmer's invention is a simple combination of a large trash bag and a candle. The person sits with the lit candle between their legs, and covers themselves, legs and all, with the trash bag. A small hole should be cut, so that the face is outside the trash bag, and the top of the head is inside. The bag traps air warmed by the candle, creating a cozy microclimate. Beeswax candles work best, burning for a long time at a high temperature.
Photo from How to Make a Trench Candle at Home
And here's a good video on Youtube, showing the trench candle-making process
News Story: Volunteers in Ukraine Make Trench Candles