The trampoline has taken 4,500 man hours to build in a cave which measures 100ft by 60ft.
Today the world's biggest underground trampoline will open at Llechwedd Slate Caverns in Blaenau Ffestiniog.
Stories of mine workers and their families will also give visitors an insight into life underground.
Wales' latest purpose-built mountain biking centre will open in Blaenau Ffestiniog later.
The centre has received European funding, which will provide a major boost for tourism in the area.
It will also be a chance to preview this weekend's British Downhill Series, a major event in the mountain biking world.
Antur Stiniog, in a transformed slate quarry at Blaenau Ffestiniog, is the only purpose-built mountain biking centre in Wales with a full uplift service.
The official opening of Antur Stiniog will be followed by the chance for riders to try out other runs at the course, which is part of the Llechwedd Slate Caverns complex.
North Wales Police has welcomed the sentencing of Derek Williams, who was given a three and a half year prison sentence for downloading child abuse images.
– Investigating officer Tim Dobson
North Wales Police welcomes today’s sentencing of Derek John Williams, following his conviction for offences of making , taking and possession of indecent images of children and possessing extreme pornography. He was placed on a sex offenders register for life and will also be subject of an indefinite sexual offences prevention order.
Incidents of this nature are taken very seriously by North Wales Police and such crimes will be robustly and positively investigated.
It is reassuring to see this reflected in the sentence passed out.
A 53-year-old man has been jailed for three and a half years for downloading thousands of images of child abuse and videos of bestiality.
Derek Williams, from Blaenau Ffestiniog, admitted charges of accessing bestiality.
Mr Recorder Timothy Petts said what Williams had done was "disgusting".
Seven years ago Williams escaped prison with a suspended sentence for similar offences.
A downhill mountain biking centre in Blaenau Ffestiniog has been chosen to host the first round of the British Championships in March.
Since opening just 18 months ago, the Antur Stiniog centre has seen over 12,000 bikers descend its 4 trails.
Organisers are anticipating around 1,000 spectators at the event, and over 350 competitors.
There's a replacement bus service running on Arriva Trains Wales between Blaenau Ffestiniog and Llandudno because of poor weather.
Today's 6.30am Arriva Trains Wales service from Blaenau Ffestiniog to Llandudno Junction has been cancelled. A member of train crew is unavailable.
Workers at a Gwynedd power station have been celebrating half a century of producing electricity today.
The plant at Blaneau Ffestiniog generates power using water from a reservoir in nearby mountains.
The electricity it makes is used to top up the National Grid when there are peaks in demand.