The Tank Must line was first introduced in the 1970s, and returns in 2021 with a brand new solar-powered edition. It's not only the first solar option in the collection, but is the first Cartier solar-powered watch ever. The slim timepiece is available in two options, a 33mm x 25.5mm and a 29.5mm x 22mm option. On average, watch owners can expect the watch to tick away faithfully for around 16 years before needing service and takes in solar power through small perforations in the dial's Roman numerals. Continuing the sustainable theme, each watch is finished with a strap made from European grown apples for a unique, non-animal leather alternative.