The National Botanic Garden of Wales (Welsh: Gardd Fotaneg Genedlaethol Cymru) is a botanical garden located in Llanarthney in the River Tywi valley, Carmarthenshire, Wales. The garden is both a visitor attraction and a centre for botanical research and conservation, and features the world's largest single-span glasshouse.

Built between 1995 and 2000, the Great Glasshouse is 95 meters long and 55 meters wide. The roof’s 785 glass panels are supported by twenty-four aluminum arches which rise to 15 meters at the dome’s apex. The dome is designed from tubular-steel supporting structures which allow light in, and a concrete substructure on the north side, partly banked by turf, provides some protection from harsh winds. 

Designed by Norman Foster and Partners, the Great Glasshouse is poised on the Welsh landscape like a giant raindrop.

It houses some of the most endangered plants on the planet which come from six areas of the world: California, Australia, the Canary Islands, Chile, South Africa, the Mediterranean Basin.

Each of these geographical areas has regions that enjoy a Mediterranean climate. Hot dry summers, cool moist winters, dazzling sunlight, strong breezes and the occasional ground-clearing fire, create perfect conditions for many plants to thrive on the scrubby, rock-strewn landscapes.

The National Botanic Garden attests that this glasshouse holds the Northern Hemisphere’s largest collection of Mediterranean plants, noting that while these six regions cover less than 2 percent of the Earth’s surface, they contain more than 20 percent of all known flowering plant species.

According to wikipedia