Morecambe Bay forms part of the Irish Sea, bordered by the English counties of Cumbria to the north and Lancashire to the east and south. 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 natural gas supply came from the bay but production is now in decline. Morecambe Bay is also an important wildlife site, with abundant birdlife and varied marine habitats.