On a grey winter day it might not look like anything special, but beneath the surface of Rockingham's Lake Richmond are some of the planet's earliest life forms.

The freshwater lake not far from the beach has recently been permanently added to Western Australia's heritage register.

But unlike historic buildings, its story goes back not just hundreds but millions of years.

Under the water by the eastern shore lie thrombolites — low, domed rocks that are living fossils and rare remnants of some of Earth's earliest forms of life.

Thrombolites look like rocks but are organic and formed from ancient microorganisms. ( Supplied: Perdita Phillips )

It is estimated the thrombolites formed six million years ago.

"They are actually microorganisms and bacteria that has all come together in a form and it is actually living," said Barbara Sciesinski, a volunteer at the Naragebup Regional Environment Centre.

"They change shape."

The environment centre sits on the western side of Lake Richmond and its members and volunteers are delighted that the lake has received an added level of protection by the State Heritage office.

"It's fantastic now that it's fully on the register, and I know a lot of the locals who have fought over many years to get it there are just absolutely over the moon," Ms Sciesinski said.

"They want to preserve this area because it's such an unusual thing as well as being a freshwater lake so close to the ocean."

The lake is home to an array of wildlife. ( ABC Radio Perth: Emma Wynne )

The lake is fed by three local drain systems, is surprisingly deep (15 metres at its deepest point), and less than a kilometre from the coast.

It was also a vital resource for the human inhabitants of the area, beginning with the Noongar people of south-west WA.

"Because that lake is fresh water, it was a meeting place for them to get fresh water, it attracts animals and birds and wildlife in general," Ms Sciesinski said.

"To them it is a meeting place and nearby at Point Peron there are also burial grounds."

Lake Richmond is protected, but surrounded by development. ( ABC Radio Perth: Emma Wynne )

After the Swan River colony was formed in 1829, early British settlers used the lake as a water source for their stock and the surrounding area for grazing.

During World War I the lake was used by the 10th Light Horse Regiment who are believed to have established an encampment there.

The state heritage register also records that "up until 1956, duck shooting was permitted, and for a brief period the Lake Richmond Ski Club used the lake for water skiing, however this was discontinued after opposition by local residents".

These days the lake is fiercely guarded by residents and the local council.

"No-one is allowed on the lake unless you have a special licence," Ms Sciesinski said.

"The ones that do have permits to use a boat on there have to display certain markings so people don't yell at them to get off the lake.

"It is being left as it is."

A boardwalk over Lake Richmond allows visitors to see the thrombolites which are visible in summer. ( ABC Radio Perth: Emma Wynne )

The local council does its best to tackle the weeds and some invasive species live in the lake including marron and even some saltwater fish that have been introduced somehow.

But the loud hum of frogs at the shore and flurry of birds shows that it remains a key natural asset.

"I think it is well looked after, we just need more awareness that it is there so the wider community can see what is going on," Ms Sciesinski said.

"When people think of Rockingham, they think more of Palm Beach and the cafes and the swimming, not so much of the natural stuff around here.

"Lots of birds are attracted there, it's quite fascinating, the amount of wildlife in Rockingham that is concentrated in the area."

A sign warns of tiger snakes and deters visitors from leaving the boardwalk. ( ABC Radio Perth: Emma Wynne )

A boardwalk has been built so the public can look at the thrombolites without causing damage, and signage discourages walking on them by warning of tiger snakes.

"We need to leave them undisturbed and try to educate people not to actually go off the boardwalk and think, 'Oh they are just rocks and water'.

"If they are damaged they will die out and we will have lost a unique thing, they won't just grow back."