Nebraska historical marker No. 68 stands on NE 12 at Martha Avenue in Newcastle. It tells the story of the bluff near the vanished river town of Ionia that smoked and gave off heat; Lewis and Clark noted it on August 24, 1804, and it was long believed to be a volcano. Part of the bluff fell into the river in 1878. The marker was put up by the Newcastle Community Betterment Committee.