Authorities are at a loss to explain how one wing of a 40-year-old apartment block on the Florida coast suddenly collapsed in the early hours of Thursday morning, resulting in deaths, injuries and leaving scores of residents still missing.

But theories are emerging.

The accident happened at about 1:30am on Thursday local time, just north of Miami Beach in the community of Surfside.

As residents slept, the north-eastern wing of the L-shaped, 12-storey apartment block "pancaked", destroying 55 of the 136 units.

CCTV footage captured by a camera on a neighbouring building shows how the inside part of the building collapsed first, dragging down the outside tower a fraction of a second later.

The catastrophic accident has resulted in multiple deaths and more than 150 other residents remain unaccounted for.

Authorities are still trying to determine how many people were in the part of the building complex that collapsed.

"There's no reason for this building to go down like that," Surfside Mayor Charles Burkett told reporters. "Unless someone literally pulls out the supports from underneath, or they get washed out, or there's a sinkhole or something like that because it just went down."

The Champlain Towers South apartment block (called a condominium, or condo, in the US) was built in 1981 in a community 20 kilometres north of Miami.

Rescue crews with sniffer dogs are continuing to comb through the mangled mess of concrete and steel, looking for signs of life.

Surfside lies on a stretch of coast that is adjacent to "Hurricane Alley", an area of warm water in the Atlantic Ocean where severe tropical storms form.

Because of this vulnerability, Florida has some of the strictest building codes in the United States.

But 70 per cent of residential towers and family dwellings in Miami-Dade county were built before those building codes were upgraded in the early 1990s, according to a report in the Miami Herald. That's more than 700,000 properties.

Concrete buildings on or near the seaside often face higher risks of structural malfunction because of their exposure to sea salt and coastal winds.

Cracks in the concrete can expose reinforcing steel rods or bars to corrosion, causing them to expand or flake and create more opportunity for water penetration, spreading the concrete degradation.

This process of spalling is also known as concrete cancer and, if left unchecked, can compromise a building's structural integrity.

For that reason, local authorities require buildings over 40 years old to undergo mandatory electrical and structural testing. That process was already underway.

But there are concerns that these inspections do not take sufficient account of subterranean damage caused by rising sea levels and the state of the bedrock.

Although there was no evidence that the building was in any imminent danger, it was about to undergo extensive repairs for rusted steel and damaged concrete, according to The New York Times.

There were also concerns that recent construction nearby may have undermined the building's structural integrity.

However, reports say the building had been built on reclaimed wetlands, which were native to this area before development took off.

And a study of the area published last year by Shimon Wdowinski, a professor at Florida International University's department of earth and environment, identified signs of land subsidence.

Satellite data showed the building, which was not named in the report, had sunk by about 2mm a year between 1993 and 1999.

Champlain Towers South is home to a mixture of permanent and seasonal residents drawn to its beachfront position, panoramic views and the region's warm climate.

A four-bedroom penthouse suite in the complex sold in May for $US2.9 million ($3.8 million), according to real estate agency reports. And a three-bedroom unit on the ninth floor sold for $US710,000 earlier this month.

Surfside is a township of about 5,600 residents located on a barrier island on the south-east coast of the Atlantic Ocean near the southern tip of Florida. It is part of Miami-Dade County and lies halfway between the cities of Miami to the south and Fort Lauderdale to the north.

The township backs onto a large lagoon, known as Biscayne Bay, and the area is dotted with homes built around canals and waterways, similar to developments found on Queensland's Gold Coast.

Barrier islands are formations created when ocean waves deposit sediment parallel to the shoreline.

Uninhabited barrier islands are constantly changing and may grow or even disappear, depending on shifting weather patterns and long-term climate change.

Like most of the beachfronts along this 1.6km strip of Atlantic coast, Surfside's golden sands are only there because of what is euphemistically called a "beach renourishment program", meaning sand had to be imported to prevent it from entirely disappearing.

Like many of Florida's beaches, the Surfside beachfront is designated by the state's Department of Environmental Protection as "critically eroded".

Image and video credits

Aerial image of the apartment collapse: AFP, Joe Raedle

A view of the scene from the beach: Reuters, Octavio Jones

The 3D view and maps of the scene were created using Google Earth Studio

Inset video of the building collapse courtesy of MSNBC