The average depth is 1,744 metres (5,722 ft) and maximum depth is 5,049 metres (16,565 ft).
Parts of the continental shelf extend far into the bay, resulting in fairly shallow waters in many areas and thus the rough seas for which the region is known. The Bay of Biscay is home to some of the Atlantic Ocean's fiercest weather. Large storms occur in the bay, especially during the winter months. Up until recent years it was a regular occurrence for merchant vessels to founder in Biscay storms, and many lives were lost. Improved ships and weather prediction have reduced the toll of the storms.
The International Hydrographic Organization defines the limits of the Bay of Biscay as "a line joining Cap Ortegal ( WikiMiniAtlas43°46′N 7°52′W / 43.767°N 7.867°W / 43.767; -7.867) to Penmarch Point ( WikiMiniAtlas47°48′N 4°22′W / 47.800°N 4.367°W / 47.800; -4.367)".
As in many west coast, the phenomenon of June Gloom is common. In late spring and early summer a large fog triangle fills the southwestern half of the bay, covering just a few kilometres inland.
As winter begins, weather becomes severe. Depressions enter from the west very frequently and they either bounce north to the British Isles or they enter the Ebro Valley, dry out, and are finally reborn in the form of powerful thunderstorms as they reach the Mediterranean Sea. These depressions cause severe weather at sea and bring light though very constant rain to its shores. Sometimes powerful windstorms form if the pressure falls rapidly, traveling along the Gulf Stream at great speed, resembling a hurricane, and finally crashing in this bay with their maximum power, such as the Klaus storm.
The Gulf Stream enters the bay following the continental shelf's border anti-clockwise (the Rennell Current), keeping temperatures moderate all year long.
The main cities on the shores of the Bay of Biscay are:
- France: Brest, Nantes, La Rochelle, Bordeaux, Bayonne, Biarritz.
- Spain: Donostia-San Sebastián, Bilbao, Santander, Gijón, Avilés.
The southern end of the gulf is also called in Spanish "Mar Cantábrico" (Cantabrian Sea), from the Estaca de Bares, as far as the mouth of Adour river, but this name is not generally used in English. It was named by Romans in the 1st century BC as Sinus Cantabrorum (Bay of the Cantabri), more frequently, Mare Gallaecum.
On some medieval maps, the Bay of Biscay is marked as El Mar del los Vascos (the Basque Sea). The Bay of Biscay is the birthplace of what is considered one of the world's most successful and most renowned maritime industries, the Basque and Cantabrian shipbuilders and fishermen.
The car ferries from Gijón to Nantes/Saint-Nazaire, Portsmouth to Bilbao and from Plymouth, Portsmouth and Poole to Santander provide one of the most convenient ways to see cetaceans in European waters. Often specialist groups take the ferries to hear more information.
Volunteers and employees of Biscay Dolphin Research regularly observe and monitor cetacean activity from the bridge of the ships on the P&O Ferries Portsmouth to Bilbao route. Many species of whales and dolphins can be seen in this area. Most importantly, it is one of the few places where the beaked whales, such as the Cuvier's beaked whale, have been observed relatively frequently. This is the best study area in the world for beaked whales.
The best areas to see the larger cetaceans lie in the deep waters beyond the continental shelf, particularly over the Santander Canyon and Torrelavega Canyon in the south of the Bay.
The three-day round trip also gives sightings of good numbers of several species of seabirds, particularly gannets.
The alga Colpomenia peregrina was introduced and first noticed in 1906 by oyster fishermen in the Bay of Biscay.
The Grammatostomias flagellibarba commonly known as the scaleless dragonfish are native to these waters.