A natural harbour is a land-form where part of a body of water is sheltered and deep enough to offer a safe anchorage.

Many of these harbours are rias, a type of estuary formed at the mouth of a river, where a valley has been flooded.

These harbours have historically played a crucial role in strategic, naval, and economic matters, making them the foundation for some of the world's most important cities. The protection provided by a naturally formed harbour reduces or eliminates the need for artificial breakwaters, ensuring calmer conditions within the harbour.

Determining the largest natural harbour is difficult due to the scarcity of precise data on the size of the contenders. Even when measurements are available, comparing them is challenging due to the irregularities and curves of different coastlines. Additionally, not all of the area within these harbours may be suitable for anchorage without constant dredging.

1. Kaipara Harbour – New Zealand

Kaipara Harbour is a vast enclosed estuary located on the northwestern side of New Zealand's North Island. At high tide, it covers an area of 366 square miles (947 sq. km), with 158 square miles (409 sq. km) exposed as mudflats and sand flats at low tide.

Spanning 37 miles (60 km) from north to south, the harbour features several arms extending into the peninsula's northeast. Formed from drowned river valleys, the shoreline is intricate, shaped by numerous rivers and streams.

The harbour head is known for dangerous sandbanks where waves from the Tasman Sea break, earning it the nickname the graveyard, due to numerous shipwrecks over the years. Today, the harbour remains infrequently used for shipping due to these hazardous conditions.

2. Cork – Republic of Ireland

Cork Harbour, located east of Cork City, is a natural harbour spanning 22km by 16km. With a water surface area of around 70 sq. miles (181 sq. km), it features Great Island, where the town of Cohn is located. This harbour's narrow opening to the sea makes it a strategically significant location.

3. Poole Harbour – Dorset, England

Poole Harbour is a large natural harbour formed as a drowned valley (ria) at the end of the last Ice Age. It covers approximately 36 sq. km (14 sq. miles) and serves as the estuary for several rivers, including the largest, the Frome.

Though the harbour has an average depth of just 48 cm and a dredged channel, its history of human settlement dates back to pre-Roman times. The harbour also features a number of islands, the largest being Brownsea Island.

4. Port of Mahon – Minorca

Port of Mahon is the largest natural harbour in the Mediterranean, stretching 3.7 miles (6 km) in length and up to 1,200 meters in width. With depths reaching 30 meters, it has been used as a port since the 3rd century BC. The natural shape of the harbour offers excellent shelter, which made it a prime location for naval fleets throughout history.

5. Falmouth & the Carrick Roads – Cornwall, England

Falmouth Harbour, part of the Carrick Roads estuary, is considered the third-largest natural harbour in the world. Its waters reach depths of up to 34 meters, making it the deepest natural harbour in Western Europe.

6. Halifax Harbour – Nova Scotia, Canada

Halifax Harbour is one of the world's deepest natural harbours, with depths of 18 meters at low tide. Formed by a drowned glacial valley, it has been a critical economic hub for over 250 years. The harbour’s strategic location made it a key point for settlement and trade.

7. Milford Haven – Pembrokeshire, Wales

Milford Haven, a ria or drowned river valley formed at the end of the last Ice Age, is one of the world's deepest natural harbours, with depths exceeding 17 meters. As the UK’s largest energy port, it handles a significant portion of the country’s oil and gas needs.

Each harbour, with its distinct features—whether shallow and intricate, or deep and expansive—continues to shape maritime activities, industries, and communities today!