Historical clue formulations

• 23-Across debut of 1999 — 2026-03-14
• Much of Greenland — 2024-07-03
• Frozen expanse — 2021-01-11
• About 10% of Russia — 2018-10-21
• Where caribou roam — 2017-09-27
• Arctic plain — 2017-05-04
• Land beyond the tree line — 2016-11-25
• Much arctic land — 2015-06-23
• Caribou's domain — 2011-03-24
• Treeless area — 2008-09-03
• Feature of northern Canada — 2003-06-01