Giant kelp is not a plant. It is an alga with no roots, no trunk and no leaves. It holds on to rock with a tangle called a holdfast, and fills gas bladders to stand up. It grows up to 60 cm a day, which makes it one of the fastest-growing organisms on Earth. A mature stand reaches 45 m from seabed to surface. Swimming into one is genuinely like entering a forest: shafts of green light, a canopy overhead moving with the swell, and things living at every level of it.
Kelp only grows in cold, nutrient-rich, well-lit water, which is why it belts the temperate coasts and skips the tropics entirely. That cold is the price of admission — most of the places below need a thick wetsuit or a drysuit, and the visibility is decided by the swell. On a good day you see 15 m; after a storm you see your own hand.
There is an honest note attached to this one. Kelp forests are in trouble in several of these places: Tasmania has lost more than 90% of its giant kelp to warming currents, and stretches of northern California collapsed after a sea urchin explosion in the 2010s. Some of the sites below are recovering, some are protected, and one or two are worth seeing partly because they may not be there in the same form in twenty years.
Where on Earth it's best
1 Signature spotMonterey Bay & Point Lobos
United States
The densest, most accessible giant kelp forest in the world, growing on rock at the head of a submarine canyon that pumps cold nutrient-rich water up into it. It is also the one with the mammals: sea otters wrapping themselves in the canopy to sleep, harbour seals threading the stipes, and sea lions using the forest as a hunting corridor. Point Lobos is a state marine reserve with a permit system that caps divers, so the forest is not loved to death.
The canyon is the reason. Deep cold water arrives at the shore here in a way it does almost nowhere else on a temperate coast.
When · Best visibility September to November, when the summer upwelling stops and the water clears. The kelp canopy is fullest in late summer.
2False Bay & Simon’s Town — the Great African Seaforest
South Africa
This is one of the few kelp forests still gaining ground: its eastern limit moved about 73 kilometres along the south coast after 2006, having held still for almost seventy years.
The only forest of its kind in the world, made of a species (Ecklonia maxima) that grows nowhere else, in water cold enough to hurt. Made famous by My Octopus Teacher, and the octopus, shysharks and pyjama sharks are genuinely there.
When · Best in the southern winter, roughly May to September, when the south-easter drops and the Atlantic side clears.
Channel Islands — Anacapa & Santa Cruz
United States
Two major ocean currents meet over these islands above a zone of persistent upwelling, and that steady push of cold, nutrient-rich water is what the kelp lives on.
A national park 20 km offshore with marine reserves that have been closed to fishing since 2003 — the clearest demonstration anywhere of what a kelp forest looks like when nothing takes anything out of it.
When · September to November for visibility; the islands are divable year-round.
Fiordland — Doubtful & Milford Sounds
New Zealand
Part of that fresh layer is engineered: since the Manapōuri power scheme opened, its tailrace into Deep Cove has more than tripled the fresh water entering Doubtful Sound.
A freshwater layer stained brown by tannins sits on top of the seawater here, cutting the light so severely that deep-water species live at 10 m. You swim down through the dark layer into a black-water forest with red coral in it.
When · Year-round; the freshwater layer is thickest after rain, which in Fiordland is most of the time.
Chiloé & the Chilean fjords
Chile
Some of the largest surviving giant kelp stands on Earth, in the Humboldt Current along southern Chile, largely unvisited by divers and increasingly harvested commercially.
When · Southern summer, December to March, for tolerable conditions.
Tasmania — Fortescue Bay & the Tasman Peninsula
Australia
The remaining stands grow under the sea cliffs of the Tasman Peninsula — 300 metres of Jurassic dolerite at Cape Pillar, immediately south of Fortescue Bay.
What is left of Australia's giant kelp. Over 90% has gone since the 1960s as the warm East Australian Current pushed south, and the surviving patches are now a listed endangered ecological community with active replanting trials.
When · Southern summer, December to April; winter water drops below 12 °C.
St Abbs & the Scottish east coast
United Kingdom
Britain's first voluntary marine reserve was declared here on 18 August 1984, and it is still run on agreement between divers, boatmen and fishermen rather than on law.
Northern Europe's kelp is a different, stiffer species (Laminaria) growing in shallow, surge-swept rock — dense, waist-high forest rather than a canopy, full of wolf-fish and nudibranchs.
When · June to September, when North Sea visibility peaks and the water reaches 13–15 °C.