Deep underground exploration and mining continued sporadically until 1918, when the last of the pumps was shut off, allowing the mines to flood up to the Sutro Tunnel Level (approximately 1,640 feet beneath Virginia City). All mining from 1920 to present has taken place above the Sutro Tunnel level. Meanwhile, through the early years of the 20th century, ownership of mineral rights along the length of the Comstock Lode was consolidated primarily into a handful of companies, most of them privately held. This structure was a departure from that of the late 1800s, when upwards of 400 mining companies operated on the Comstock, with at least 150 of them publicly traded.