One of the countries most affected by the 2008 global financial crisis was Spain, where a decades-long housing boom went bust almost overnight, bankrupting developers and leaving unfinished housing projects littered across the country, from the suburbs of Madrid to the Mediterranean coast.

Ten years later, the Spanish economy and housing market have recovered, but these ghost developments remain, too unprofitable to sell and too expensive to demolish. Across the country, an estimated 3.4 million houses remain unoccupied. Many are now owned by large banks, which seized them after their original developers defaulted on their loans.

Basque photographer Markel Redondo first photographed these developments in 2010, shortly after the bust, for a series called Sand Castles. He recently revisited many of them for a second series, Sand Castles II, intended to mark the 10-year anniversary of the financial crisis. Most of the developments are guarded only by a ring of fencing; thieves have long since stripped out everything of value, from the copper wiring to the manhole covers on the access roads. "You have to drive carefully, because there are holes everywhere," Redondo says.

For the new series, Redondo used a drone to capture stunning aerial views of the half-finished developments, which often look like the ruins of an abandoned civilization. Because many of the developments were built in semi-remote locations, many Spaniards have forgotten they exist; part of Redondo's goal is to remind his fellow countrymen of the consequences of unchecked real estate speculation.

"The mistakes Spain made haven't really stuck in people minds," he says. "They talk about the crisis being over, that the economy is back, but I wanted to look back at these mistakes so we don't forget them. And I want us to do something about these places, because they're not good for the environment."

Paradoxically, Spain is currently experiencing yet another housing boom, with the number of homes sold or purchased reaching the highest level since 2008. And like the rest of Europe, it has seen an influx of refugees from the Middle East and Africa. To Redondo, the solution seems obvious.

"We could do a lot of things with these [abandoned projects]," he says. "People are being kicked out of their houses in every major city because they can't afford them. We have a lot of people who need homes. We should do something with them, not just leave them there or destroy them."

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