Dallas firefighters are battling an underground blaze in the heart of downtown.

Crews found flames blowing through a grade in the sidewalk just after 3 a.m. Friday morning. As they battled the blaze, plumes of thick smoke rose into the air outside a popular hotel on Main Street.

Firefighters managed to contain the flames to an electrical vault, but couldn’t extinguish it entirely until an Oncor crew shut the power down. According to the electric company, the fire cut power to five blocks of downtown Dallas, including city hall, for nearly five hours.

The cause of the fire is still under investigation.