diff --git a/index.html b/index.html
index 9e53332..2847df4 100644
--- a/index.html
+++ b/index.html
@@ -55,8 +55,34 @@
 <header>
   <div class="container">
     <span class="header-skull" aria-hidden="true">💀</span>
-    <h1 class="site-title glow-text">AI DEATH CLOCK</h1>
-    <p class="site-subtitle">The True Environmental Cost of Global AI Token Consumption</p>
+    <h1 class="site-title glow-text">HUMAN EXTINCTION COUNTDOWN</h1>
+    <div class="extinction-timer" id="extinctionTimer" aria-live="polite" aria-label="Human extinction countdown">
+      <div class="extinction-unit">
+        <span class="extinction-value" id="extYears">—</span>
+        <span class="extinction-unit-label">YRS</span>
+      </div>
+      <span class="extinction-sep" aria-hidden="true">:</span>
+      <div class="extinction-unit">
+        <span class="extinction-value" id="extDays">—</span>
+        <span class="extinction-unit-label">DAYS</span>
+      </div>
+      <span class="extinction-sep" aria-hidden="true">:</span>
+      <div class="extinction-unit">
+        <span class="extinction-value" id="extHours">—</span>
+        <span class="extinction-unit-label">HRS</span>
+      </div>
+      <span class="extinction-sep" aria-hidden="true">:</span>
+      <div class="extinction-unit">
+        <span class="extinction-value" id="extMins">—</span>
+        <span class="extinction-unit-label">MINS</span>
+      </div>
+      <span class="extinction-sep" aria-hidden="true">:</span>
+      <div class="extinction-unit">
+        <span class="extinction-value" id="extSecs">—</span>
+        <span class="extinction-unit-label">SECS</span>
+      </div>
+    </div>
+    <p class="extinction-caption">at current global AI token consumption rate &middot; <a class="extinction-link" href="#milestones-section">see milestones ↓</a></p>
   </div>
 </header>
 
diff --git a/src/js/02-counter.js b/src/js/02-counter.js
index b326c3f..b71765c 100644
--- a/src/js/02-counter.js
+++ b/src/js/02-counter.js
@@ -158,3 +158,57 @@
     if (el) el.textContent = value;
   }
 
+  // ---- Extinction countdown (header) -----------------------
+  // Module-level constants and helpers so they are created once, not on every
+  // call to updateExtinctionCountdown() (which runs every second).
+  const _EXT_SECS_PER_YEAR = 365.25 * 24 * 3600;
+  const _EXT_SECS_PER_DAY  = 24 * 3600;
+  const _EXT_SECS_PER_HOUR = 3600;
+  const _EXT_SECS_PER_MIN  = 60;
+  const _EXT_UNIT_IDS = ['extYears', 'extDays', 'extHours', 'extMins', 'extSecs'];
+  const _extPad2 = (n) => String(Math.floor(n)).padStart(2, '0');
+  const _extPad3 = (n) => String(Math.floor(n)).padStart(3, '0');
+
+  // Finds the milestone flagged `extinctionMarker: true`, computes the
+  // remaining seconds at the current dynamic rate, and updates the header
+  // countdown display once per second.
+  function updateExtinctionCountdown() {
+    const extinctionMilestone = MILESTONES.find(m => m.extinctionMarker);
+    if (!extinctionMilestone) return;
+
+    const tokens = getCurrentTokens();
+    const currentRate = getDynamicRate(new Date());
+    const tokensRemaining = extinctionMilestone.tokens - tokens;
+
+    if (tokensRemaining <= 0) {
+      // Extinction threshold reached — show zeroed-out timer
+      _EXT_UNIT_IDS.forEach(id => {
+        const el = document.getElementById(id);
+        if (el) el.textContent = '00';
+      });
+      return;
+    }
+
+    const secsRemaining = tokensRemaining / currentRate;
+    const years    = Math.floor(secsRemaining / _EXT_SECS_PER_YEAR);
+    const remAfterY = secsRemaining % _EXT_SECS_PER_YEAR;
+    const days     = Math.floor(remAfterY / _EXT_SECS_PER_DAY);
+    const remAfterD = remAfterY % _EXT_SECS_PER_DAY;
+    const hours    = Math.floor(remAfterD / _EXT_SECS_PER_HOUR);
+    const remAfterH = remAfterD % _EXT_SECS_PER_HOUR;
+    const mins     = Math.floor(remAfterH / _EXT_SECS_PER_MIN);
+    const secs     = Math.floor(remAfterH % _EXT_SECS_PER_MIN);
+
+    const yrsEl  = document.getElementById('extYears');
+    const daysEl = document.getElementById('extDays');
+    const hrsEl  = document.getElementById('extHours');
+    const minsEl = document.getElementById('extMins');
+    const secsEl = document.getElementById('extSecs');
+
+    if (yrsEl)  yrsEl.textContent  = String(years);
+    if (daysEl) daysEl.textContent = _extPad3(days);
+    if (hrsEl)  hrsEl.textContent  = _extPad2(hours);
+    if (minsEl) minsEl.textContent = _extPad2(mins);
+    if (secsEl) secsEl.textContent = _extPad2(secs);
+  }
+
diff --git a/src/js/21-boot.js b/src/js/21-boot.js
index 2de1dac..be5a48c 100644
--- a/src/js/21-boot.js
+++ b/src/js/21-boot.js
@@ -69,10 +69,14 @@
     // Kick off the live counter RAF loop
     requestAnimationFrame(updateCounters);
 
+    // Populate extinction countdown immediately, then refresh every second
+    updateExtinctionCountdown();
+
     // Check time-based badges and milestone alert every second
     setInterval(() => {
       checkTimeBadges();
       checkMilestoneAlert();
+      updateExtinctionCountdown();
     }, 1000);
   }
 
diff --git a/styles/hero-tabs.css b/styles/hero-tabs.css
index 198c094..af2cc20 100644
--- a/styles/hero-tabs.css
+++ b/styles/hero-tabs.css
@@ -33,11 +33,80 @@ header::after {
   text-shadow: 0 0 20px var(--accent-glow);
 }
 
-.site-subtitle {
+/* ---- Extinction Countdown Timer ---- */
+.extinction-timer {
+  display: flex;
+  justify-content: center;
+  align-items: flex-start;
+  gap: 0.15rem;
+  margin-top: 0.75rem;
+  flex-wrap: wrap;
+}
+
+.extinction-unit {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  min-width: 3rem;
+}
+
+.extinction-value {
+  font-family: 'Orbitron', monospace;
+  font-size: clamp(1.8rem, 5vw, 3rem);
+  font-weight: 900;
+  color: var(--accent);
+  text-shadow: 0 0 18px var(--accent-glow);
+  line-height: 1;
+  font-variant-numeric: tabular-nums;
+  letter-spacing: -0.02em;
+}
+
+.extinction-unit-label {
+  font-family: 'Orbitron', monospace;
+  font-size: 0.55rem;
+  letter-spacing: 0.18em;
   color: var(--text-dim);
-  font-size: 1rem;
-  margin-top: 0.5rem;
-  letter-spacing: 0.1em;
+  text-transform: uppercase;
+  margin-top: 0.25rem;
+}
+
+.extinction-sep {
+  font-family: 'Orbitron', monospace;
+  font-size: clamp(1.4rem, 4vw, 2.4rem);
+  font-weight: 900;
+  color: var(--accent);
+  opacity: 0.35;
+  line-height: 1;
+  padding-top: 0.05em;
+  align-self: flex-start;
+}
+
+.extinction-caption {
+  font-size: 0.7rem;
+  color: var(--text-dim);
+  letter-spacing: 0.08em;
+  margin-top: 0.75rem;
+  text-transform: uppercase;
+}
+
+.extinction-link {
+  color: var(--accent);
+  text-decoration: none;
+  opacity: 0.7;
+}
+
+.extinction-link:hover {
+  opacity: 1;
+  text-decoration: underline;
+}
+
+@media (max-width: 480px) {
+  .extinction-unit {
+    min-width: 2.25rem;
+  }
+  .extinction-unit-label {
+    font-size: 0.48rem;
+  }
 }
 
 /* ---- Tab Bar ---- */
diff --git a/tests/e2e/death-clock.spec.js b/tests/e2e/death-clock.spec.js
index 014829c..b3545fb 100644
--- a/tests/e2e/death-clock.spec.js
+++ b/tests/e2e/death-clock.spec.js
@@ -41,9 +41,16 @@ test.describe('AI Death Clock — end-to-end', () => {
     await expect(page).toHaveTitle(/Token Deathclock/i);
   });
 
-  test('renders main header', async ({ page }) => {
+  test('renders main header with extinction countdown', async ({ page }) => {
     await expect(page.locator('h1.site-title')).toBeVisible();
-    await expect(page.locator('h1.site-title')).toContainText('AI DEATH CLOCK');
+    await expect(page.locator('h1.site-title')).toContainText('HUMAN EXTINCTION COUNTDOWN');
+    // Countdown timer should be present and populated after init
+    await page.waitForFunction(() => {
+      const el = document.getElementById('extYears');
+      return el && el.textContent.trim() !== '—' && el.textContent.trim() !== '';
+    }, { timeout: 3000 });
+    const years = await page.locator('#extYears').textContent();
+    expect(parseInt(years, 10)).toBeGreaterThan(0);
   });
 
   // ── Live counters ─────────────────────────────────────────────────────────