One of the world's most enduring toys lasts longer than even its makers intended.
Lego, scientists have found, could take 1,300 years to decompose.
Researchers testing Lego retrieved during beach clean-ups found its tough plastic bricks in great condition despite decades of wear and tear from constant exposure to water, sunlight and abrasive sands and stones.
The Lego tested could be dated to the 1970s because the bricks contained certain chemical colourants outlawed after that time.
Some were almost pristine, having lost as little as 3pc of their mass, while the most severely degraded had lost no more than 40pc.
That means, according to the team from the University of Plymouth in Britain, that at best it could take 100 years, and at worst 1,300 years, for the toy to decompose.
For households with children in Ireland, where Lego is a constant top-seller, that means an ever-growing plastic mountain is accumulating that will far out-last the homes it was played in.
The fact that plastic is made from petroleum and the world is in a race against time to reduce fossil fuel consumption, also casts a shadow over the playtime favourite.
Mindy O'Brien, co-ordinator of Voice of Irish Concern for the Environment, is an ardent campaigner against excessive use of plastics but she winced on hearing the study results.
"I really don't want to demonise Lego. I bought it, my kids used it, we passed it on for other kids to use it. It's a great toy," she said.
"Of course, one of the reasons we think it's great is because it's indestructible, it lasts forever, and now one of the problems with it is that it lasts forever."
The dilemma is not lost on Lego either. The Danish company states it wants its toys to last for generations so it does not want to switch to biodegradable materials, but it is working on finding environmentally-sustainable plastic alternatives.
One of those is sugar cane, which it has begun using to make polyethylene for a small number of components.
"The most challenging mission before us is to make all core Lego products from sustainable materials by 2030," the company said.
"We are continuing to experiment with many different materials as we expect the Lego bricks of the future could be made from both plant-based and recycled sources."
Other major toy manufacturers, such as Mattel, have made similar pledges though others, such as Hasbro, are not committing to any particular date.
In the meantime, Ms O'Brien said responsible use and re-use was the best approach.
"Our biggest problem is still single-use plastics. That still forms the bulk of what we find on beaches. But if you're buying Lego, mind it, enjoy it, pass it on and keep it in circulation.
"Don't just dump it if you have no need it for it and remember, no plastic should end up in the sea," she said.
The Plymouth study, as reported in 'Science News', used specialist X-rays to examine the bricks which were gathered during beach clean-ups along the Cornwall coast.
Report author Dr Andrew Turner told the publication the importance of the research stretched way beyond Lego as it showed the kind of challenges presented by all sorts of consumer and electronic goods that were similarly made of hard, durable plastics.