Pioneer Canyon has steep-sided gorges on the seafloor of the continental slope, west of San Mateo County, CA. Pioneer Canyon is approximately 48 kilometers (30 miles) long, at its widest point is 3.7 kilometers (2.3 miles) wide, and ranges in depths from -200 meter depth to -2500 meters depth. The head of the canyon is approximately 32 kilometers (20 miles) west of Half Moon Bay, off of the San Mateo County coast. The eastern half of the canyon, including the head of the Canyon, is within Monterey Bay National Marine Sanctuary (24 km, from -200 m to -1350 m in depth). Staff from Greater Farallones National Marine Sanctuary administers this portion of Monterey Bay National Marine Sanctuary.
Bamboo and black corals have long been thought to occur in Pioneer Canyon, having been described in dive records from the submersible Sea Cliff in the late 1980s and early 1990s and listed as bycatch in fishing trawl records. In 2016, GFNMS partnered with NOAA Office Ocean Exploration and Research and Ocean Exploration Trust to explore Pioneer Canyon using their deep sea reproach class ROV Hercules. The high definition camera on Hercules revealed many bamboo coral forests and rocky features with complex and diverse corals, sponges, other invertebrates, and associated fish. Toward the end of the dive, we found a series of small munitions dumps, including disarticulated piles of bullets and ammunition boxes. We suspect that after WW II, ships heading toward the ammunition dump far west of the canyon prematurely dumped their ammunition along this portion of the canyon.