Under the surface of the River Derwent you'll find an otherwise hidden and surprising colourful world of marine creatures.

Thousands of people drive over the Tasman Bridge every day, but not that many dive under it.

The river plummets 30 to 40 metres down right near the shore in the Derwent. ( Supplied: Millie Banner )

Millie Banner has been diving for about three years and recently did a dive under the Tasman Bridge.

"When we first stuck our heads in the water it was just green," she said.

"The visibility was terrible and we had no idea it would be so beautiful.

Wreckage from the Tasman Bridge collapse in 1975 has created a haven for marine life. ( Supplied: Millie Banner )

"There's all the wreckage from the bridge from when it collapsed ... it's just beautiful.

"There's all the old pylons, there's a lot of just stuff down there.

"If there wasn't all that wreckage, there probably wouldn't be anything down there."

All that stuff means jewelled anemones, crayfish and fish have places to thrive 40 metres beneath the surface.

Ms Banner took photographs of what she saw.

Jewel anemones provide a surprising burst of colour in the murky waters of the Derwent. ( Supplied: Millie Banner )

"I love being able to take photos and take video and just share with so many people who otherwise wouldn't see that kind of thing.

"If I can share my photos ... just get the word out and start understanding what's there, we can appreciate it and start making sustainable developments towards cleaning up the river."

Ms Banner is studying at the University of Tasmania and works as a supervisor with the Green Army on a conservation project.

Millie Banner hopes to make a career with her photography and diving. ( ABC Radio Hobart: Carol Rääbus )

She hoped to one day be able to combine her love of conservation and diving in a future career.

"I love working outdoors, getting in the environment and making a bit of a difference.

"I would love to do something with my photography and diving."

In the meantime, Ms Banner will keep diving around Tasmania to capture worlds most of us never see.

She shares what she finds on her Facebook page My Bubble Photography.