The drone, a student-built project, let the experts take a look inside the sinkhole "without putting people at harm," Dettman says. It also "helped confirm" theories about the sequence of events.

Working With Nature

Engineers were finalizing plans for filling the sinkhole in late February. Dettman says traditional sinkhole repairs involve inserting a large rock, then layers of gradually smaller rock.

"That sinkhole is breathing, and you want it to continue to be that drain, taking surface water and draining it into the cave," which the drone showed running off one side of the sinkhole, he says.

A filter fabric will keep the soil from washing out through the rock and destabilizing the area again, Dettman says.

Stormwater management is important at a site such as this, where the atrium is surrounded by large gravel. Crews have placed plastic around the structure to divert rainwater from the building to a nearby retention pond, he says.

The site also may undergo an electric resistivity study, a technology that "has rapidly improved over the last 10 years," Dettman says. Investigators transmit an electric current through a series of ground probes. "Highly dense material will show up as green and air as red," he says.

The atrium has been closed off, but crews have built a glass wall so observers can watch the stabilization and repair work as it occurs.