Why does Ladybower Reservoir in Derbyshire have plug holes?

Two fishermen caused a stir earlier this week when they were filmed sailing perilously close to a giant "plug hole" at Ladybower Reservoir in Derbyshire.

Severn Trent Water, which owns the reservoir, said the pair had put themselves in a serious danger and warned others not to copy them.

Ladybower Reservoir has two of these dramatic-looking "plug holes" but exactly what do they do?

One wag on social media joked the plug holes provided a direct route to Australia while another person said they were told as a child there was a giant mincing machine at the bottom.

Inevitably the truth is more mundane.

Technically known as shaft spillways but affectionately known as plug holes, the drains were designed to regulate water levels in the reservoir by letting out water when the reservoir became full after prolonged wet spells.

The water is carried away down tunnels through the dam to the River Derwent downstream.

Each plug hole has a diameter of 24m (78ft) and the drop from the exit point to the floor of the tunnel below is 20m (66ft).

These shaft spillways have been nicknamed bellmouth or morning glory - after the flower - spillways.

Importantly, they look pretty cool too.

Following the water down the hole would not end well, according to Severn Trent Water.

"If somebody fell in, they would very seriously hurt themselves," a spokeswoman said.

"They would also find themselves stuck because access to the river has a grille so they would need a professional rescue team to get them out."

Good question. These plug holes aren't just about getting rid of excess water - they also create energy.

Severn Trent Water has proudly stated that water released from the reservoir is used to create hydroelectric energy to power its water and sewage treatment works.

You might think the overflows would be the perfect place for this to happen, but the firm said it actually takes place inside the dam, with excess water from pipework passing through two large turbines.

Absolutely, here you go.

Yes they do - but not many. Next time you're enjoying a visit to the Brecon Beacons in Wales, look out for one at Llyn Pontsticill Reservoir.

You'll find another near the Monticello Dam in Lake Berryessa, Napa County, California. Its nickname is "The Glory Hole" if you need to ask a local for directions.

A more common sight than shaft spillways are overflow spillways, which carry water down part or all of the dam.

It was damage to the spillway at Toddbrook Reservoir in Whaley Bridge, Derbyshire, that saw more than 1,500 residents ordered to leave their homes over fears the dam would collapse and flood the town.

Other reservoirs employ side channel spillways, which as the name suggests channel the water away from the side of the dam, or outlet towers, which mechanically pump water from the reservoir.

You'll find one of these at Rutland Water along with a limnological tower - used to study aquatic ecosystems within the reservoir.

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