Rescue crews are battling dangerous situations in a race against the clock to find people who may be trapped under the giant mountain of debris from the Surfside, Fla., condominium building that collapsed early Thursday.

Search and rescue teams are surveying the outer portions from above, others are tunneling into the pile from below, and heavy machinery is being brought in to lift some of the material from the top of the rubble.

Different Kinds of Collapses

The collapse compressed a 12-story building into a pile estimated to be about 20 to 30 feet high, or the equivalent of about 2 or 3 stories tall. Much of the building appears to have fallen in “pancake,” V-shaped and lean-to patterns, according to Scott Goldstein, Fire Chief of the Montgomery County, Md., Fire and Rescue Service, who reviewed photos and videos from the scene.

In a pancake collapse, horizontal slabs of concrete and debris fall directly on top of one another, leaving little space in between for survivors. There are also likely to be V-shaped collapses (or so-called lean-to ones, which are similar to a V-shaped, but with one slab instead), where pieces of ceiling or structurally strong pieces of furniture fall at an angle, creating voids, or larger spaces where people can survive for several days or longer, Mr. Goldstein said.

“Throughout the world, time and time again, we see stories of people who survive multiple days, up to seven, sometimes 10, in a building that has collapsed,” Mr. Goldstein said. “That’s because they were in a void space. That was because they weren’t physically crushed, or partially crushed by the structure.”

Survivors are most likely to be found in those void spaces within the rubble, Mr. Goldstein said. After initially surveying the top of the debris for victims who would have been easily accessible, search and rescue teams focused their efforts underground, working through the parking garage.

Searching for Voids in the Mountain of Debris

At one point Friday, 90% of the rescue operations were happening underneath the building, according to Raide Jadallah, assistant chief for Miami-Dade Fire Rescue.

The sloped ceiling of the parking garage where they worked indicates a V-shaped or lean-to collapse, Mr. Goldstein said, with a chance there is a void space behind it where there may be survivors, or it may lead to another slab they can cut through to go deeper in.

Rescue workers typically drill holes in the concrete to find the next void space, removing smaller chunks of debris as they go and shoring up the pile with temporary braces to avoid major shifts or further collapse that could endanger the rescue workers or those who may be underneath.

Often, fires pop up, hindering efforts to penetrate deeper. Crews put them out but try not to use much water, as the additional weight from the water pushes down on the debris and can cause it to shift.

As of Friday, thunder and pouring rain rolled in waves over the scene. A cloud of dust and the stench of fire hung over the area. In the coming days, the rescue teams plan to use cranes to begin removing heavy debris from the top of the pile to aid in the search for possible survivors.

—Scott Calvert and Kara Dapena contributed to this article.

As the search for survivors entered its fourth day, the death toll rose to nine in the 12-story condo tower in Surfside, Fla. that partially collapsed. Officials pressed ahead with efforts to investigate the cause of the failure and assess the safety of nearby buildings. Photo: Cristobal Herrera-Ulashkevich/Shutterstock The Wall Street Journal Interactive Edition

Write to Brian McGill at Brian.McGill@wsj.com