A portion of the north side of San Diego’s Black Mountain Open Space Park was closed last Friday after city officials found high levels of arsenic around an abandon mine.

The city temporarily closed most of the Miner’s Ridge Loop Trail, leaving open only the Glider Point Trail section of the loop. The mine and surrounding areas are off limits to the public. Further testing is expected to take several months, so expect the trail to be closed at least until then.

The city released a new trail map of the area showing the temporary closure:

blackmountaintrailclosure

The mine, which dates back to the 1920s, was established to extract white arsenic, which to eradicate boll weevils on Southern cotton farms. The project was headed by Frank Hopkins, an Escondido rancher, rodeo cowboy and actor, who leveraged his friends in Hollywood to finance the mine.

It was mined for less than a decade before being abandon after several cold winters and dry summers wiped out the boll weevils, resulting a crash in the demand for arsenic. The mine hasn’t operated since 1927, but mine shafts, an oven and other mining ruins remain.