WATCH: See inside giant Victorian underground reservoir in Norwich

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Despite being hugely important to day-to-day life in Norwich, the 150-year-old Lakenham reservoir spends most of its life undisturbed by humans.

But every 10 years inspectors from Anglian Water climb down into the depths to drain, examine and clean the reservoir - and Wednesday, October 16 was the day chosen for this decade's inspection.

The reservoir is not open to visitors, so these videos and pictures are a unique opportunity for the people of Norwich to explore one of the city's architectural marvels.

Nik Shelton, from Anglian Water, labelled it an "absolutely beautiful structure" which was still in great condition despite its ripe age.

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

"When it was built it was an absolute feat of Victorian engineering," he said.

"Once every 10 years we drain it down and we give it a good thorough inspection, a real thorough clean and then it gets filled back up again and goes back into supply, so [this is] a very rare opportunity to have a look inside an amazing bit of Victorian engineering underneath Norwich."

Built in 1871, the Lakenham reservoir is the basin into which all Norwich's water pours after it has come from the River Wensum and been treated at the Heigham Water Treatment Works, and from which it is pumped out to supply the city.

It was built to, and still does, supply Norwich's residents with drinking water.

Neil Adcock, left, reservoir inspecting engineer, and Ed Richardson, supply support manager, under the stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Mr Shelton said the cleanliness of the water helps keep the structure relatively clean, but that regular inspections and deep cleans were important to keep it "in tip-top condition".

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Neil Adcock, left, reservoir inspecting engineer, and Ed Richardson, supply support manager, under the stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

The outside of Anglian Water's Victorian underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

The plaque from 1871 when the Victorian underground reservoir in Norwich was built. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

The hatch to Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Stunning archways and brickwork in Anglian Water's Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)

Lottie Williams from Anglian Water, studies the brickwork In the Victorian treated water underground reservoir in Norwich, which has been emptied for inspection. Picture: DENISE BRADLEY (Image: Copyright: Archant 2019)