Officials in Allentown said the hole spotted by a resident at about 1 p.m. Saturday prompted evacuation of a block of row homes.
Fire Chief Robert Scheirer said officials hope to be able to shore up and repair the homes, but residents would probably be displaced for a few days or longer.
Engineers will send a robotic camera down to see how deep the hole is and test how fast it's spreading.
In December a water main break and a sinkhole elsewhere forced two dozen Allentown residents to evacuate.
In 1994, a sinkhole damaged the city's Corporate Plaza building, which had to be demolished.