At Iwo Jima, Okinawa, and other island bases of the Japanese forces, the Japanese created a network of deep tunnels and bunkers that made US attempts to evict Japanese forces extremely difficult, time consuming and expensive in terms of loss of lives. Based on these experiences, U.S. military planners anticipated that the human cost of invading Japan would be severe. See Giangreco, D.M., "Casualty Projections for the U.S. Invasion of Japan, 1945-1946: Planning and Policy Implications," Journal of Military History, 61 (July 1997), pp. 521-582, at pp. 534-35.
Given that MacArthur planned an amphibious assault against the Japanese homeland at Kyushu, an obvious choice that the Japanese expected, did the Japanese create defense positions similar to those at Iwo Jima at Kyushu, and if so, when did they begin work how much progress had they made on these defenses by the time the atomic bomb was dropped on Hiroshima?