The southern part of these plains (including the Awarua Plains along the coast east of Bluff) contains much wetland and swamp. In the far southeast of Southland rises the rough hill country of The Catlins. This area is divided between Southland and the neighbouring Otago Region, with the largest settlement, Owaka, being within Otago. The hills of The Catlins form part of a major geological fold system, the Southland Syncline, which extends from the coast northwestward, and include the Hokonui Hills above Gore. Off the coast of Southland lies the Great South Basin which stretches over 500,000 km2 (covering an area 1.5 times New Zealand's land mass). It is one of the country's largest undeveloped offshore petroleum basins with prospects for both oil and gas. Weather conditions in Southland are cooler and wetter than the other regions of New Zealand due to its distance from the Equator. However, they can be broken down into three types: the temperate oceanic climate of the coastal regions, the continental climate of the interior and the wetter mountain climate of Fiordland to the west. Due to its closer proximity to the South Pole, the Aurora Australis or "Southern Lights" are more commonly seen than in other regions. The coastal regions have cool summers and mild winters. The mean daily temperature varies from 5.2 °C in July to 14.9 °C in January. Rainfall varies from 900 mm to 1300 mm annually with rainfall being more frequent in coastal areas and rainbows being a regular occurrence in the region. Summers are temperable with downpours and cold snaps not being uncommon. On 7 January 2010, Invercargill was hit by a hail storm with temperatures plummeting rapidly from 15 °C to 8 °C in the afternoon. Occasionally, temperatures exceed 25 °C with an extreme temperature of 33.8 °C having been reached before in Invercargill in 1948 and 35.0 °C in Winton in 2018. Winters are colder and more severe than other regions. The mean maximum temperature in July is 9.5 °C and Southland's lowest recorded temperature was −18 °C in July 1946. Snow and frost also frequently occur in inland areas but are less common and extreme in coastal areas where the oceans act as a moderating factor. The long-lasting cool and wet conditions are influenced by the presence of a stationary low-pressure zone to the southeast of the country.