Morecambe Bay is a large estuary in northwest England, just to the south of the Lake District National Park. It is the largest expanse of intertidal mudflats and sand in the United Kingdom, covering a total area of 120 sq mi (310 km²). In 1974, the second largest gas field in the UK was discovered 25 mi (40 km) west of Blackpool, with original reserves of over 7 trillion cubic feet (tcf). At its peak, 15% of Britain's gas supply came from the bay but production is now in decline. It is also one of the homes of the high brown fritillary butterfly.