Crews quickly filled in the hole, but the mishap is raising questions about the project.
Meanwhile, state Transportation Department officials say they always knew something like this was possible in the particular area where it happened, under King Street.
The sinkhole was 15 feet long, 20 feet wide and 7 feet deep, and it formed in a secure area blocked off from the public.
State DOT officials say this is the only area where Bertha will be digging through loose soil. For the first time in her journey, she's digging through soil that hasn't been strengthened with concrete.
Some of the soil along King Street is glacial till, which is dense and good for tunneling. But a layer near the surface is loose fill material, dumped by Seattle's early settlers.
The fill includes loose soil, sawdust and timber piles that - if disturbed during tunneling - can create voids above the machine.
DOT officials say more sink holes are possible, but only for another block north to Jackson Street.
After that point the dig goes deeper, and the soil is more solid.