Penelope Powell (14 October 1904 - 1 October 1965) was a pioneering cave diver. She was Diver No. 2 for the first successful cave dive using breathing equipment in Britain at Wookey Hole Caves in the Mendip Hills, Somerset on 18 August 1935. Powell was posthumously entered into the Women Divers Hall of Fame (WDHOF) in 2023.

Born Penelope Margaret Hopper to Dr Leonard Bushby Hopper and Mabel Elizabeth Jackson in Kessingland, Suffolk, she was the eldest of three children. By 1911 the family was living in Cornwall. She was niece to Rev. C. F. Metcalfe, an early explorer of the Mendip caves.

In 1927 she moved to Malaysia where she married Reginald Oliver Tyndale-Powell at St. George's Church, Penang. They had two children. She returned to the UK in 1934, where they divorced. She was living in Priddy, Somerset, and working at Wookey Hole Shop and Caves at the time of her cave dive the following year. Subsequently, she worked at Gough's Cave in Cheddar Gorge.

In 1936 Powell moved to Bristol and married John "Jock" Baxter Wyllie, a Clifton College housemaster. They had three children. In 1938 the couple set up Tankerton School, a preparatory school for boys and a mixed kindergarten, in Mundon, Essex, where special attention was given to pupils' "health and character". Following this they lived successively in Wareham, Bodmin, Tregiffian and finally Bryher, Isles of Scilly, where Powell lived until her death from breast cancer in 1965.

While working at Wookey Hole, Powell became fascinated with the "archaeological prospects" that were becoming more apparent as a result of advances in diving. She was a strong swimmer and had an "adventurous disposition". She was readily accepted into a newly formed diving group, including leader Graham Balcombe, Frank Frost, Bill Bufton, Bill Tucknott and ‘Digger' Harris, all members of the Mendip Nature Research Society.

In 1934 Balcombe and fellow diver, Jack Sheppard had made a first attempt at exploring Swildon's Hole, the longest cave in the Mendips. However, a lack of proper equipment forced them to find more suitable gear which came in the form of standard Royal Naval diving dress, including copper helmet, airline and pumps, on loan from Siebe Gorman. The following year they engaged an amateur dive team of "similarly minded individuals" including Powell, with the intention of completing the longest cave dive in British history to that date.

In July 1935 the team learnt to dive at Priddy Pools where there was both sufficient water and privacy. Powell was third in the diving order for the first attempt and remained submerged for 30 minutes. Seven preparatory dives through Wookey Hole, part of which was opened as a show cave in 1927, laid the groundwork for the main dive.

In an article published in 1953, Balcombe wrote "In the summer of 1935, by the kindness both of the diving firm who lent us the gear and taught us how to use it and of the cave proprietor who gave us every facility, we were able in a series of weekend operations to acquire experience in working underwater which set our feet along the road to conquest."