A truck hauling oil drilling waste flipped on a ramp, spilling thousands of gallons of the material and causing a big mess and a major closure.
The truck flipped on the ramp from I-35 North to 410 North just before 10 a.m. Sunday.
The thick, black material covered both lanes of 410 North and the onramp, forcing police to shut down the area to motorists.
A police sergeant on the scene said the truck driver apparently tried to take the ramp to 410 too fast and lost control.
His rig flipped and his messy cargo spilled out onto 410 and 35 on San Antonio's southwest side.
Before the cleanup could begin, TxDot had to test the material to ensure it wasn't hazardous to workers and that it would not damage the road surface.
Once the material was deemed safe, a cleaning company moved in with heavy equipment and power washers to begin clearing the material which consisted of a mixture of chemicals and dirt used in drilling operations.
Authorities diverted traffic to the I-35 Northbound turnaround at New Laredo Highway.
The stretch of road affected by the spill is not expected to reopen until Monday morning.
The driver of the truck was not injured in the accident.