Without quite walking on water, Helen Mark uses a very low spring tide to walk between Tresco and Bryher, two of the Isles of Scilly. She meets people who delight in what is revealed on the seabed, such as 3,000 year old Neolithic field walls, indicating the time when the islands were a single landmass.

Helen hears how it's all possible because of what must surely be one the best words to have up your sleeve during a game of Scrabble: syzgy, which is when the sun, moon and earth are aligned creating the extreme highs and lows known as spring tides.

Producer: Mark Smalley.