diff --git a/2021/12/go.mod b/2021/12/go.mod
new file mode 100644
index 0000000..40c9284
--- /dev/null
+++ b/2021/12/go.mod
@@ -0,0 +1,9 @@
+module AoC/2021/12
+
+require AoC/2021/common v0.0.0
+
+require github.com/joho/godotenv v1.4.0 // indirect
+
+replace AoC/2021/common v0.0.0 => ../common
+
+go 1.17
diff --git a/2021/12/go.sum b/2021/12/go.sum
new file mode 100644
index 0000000..8c9f290
--- /dev/null
+++ b/2021/12/go.sum
@@ -0,0 +1,2 @@
+github.com/joho/godotenv v1.4.0 h1:3l4+N6zfMWnkbPEXKng2o2/MR5mSwTrBih4ZEkkz1lg=
+github.com/joho/godotenv v1.4.0/go.mod h1:f4LDr5Voq0i2e/R5DDNOoa2zzDfwtkZa6DnEwAbqwq4=
diff --git a/2021/12/main.go b/2021/12/main.go
new file mode 100644
index 0000000..e486afb
--- /dev/null
+++ b/2021/12/main.go
@@ -0,0 +1,144 @@
+package main
+
+import (
+	aoc "AoC/2021/common"
+	"bufio"
+	"strings"
+)
+
+type cave struct {
+	large bool
+	connected []string
+}
+
+func connect(system map[string] cave, a string, b string) map[string] cave {
+	if _, exists := system[a]; !exists {
+		c := cave{strings.ToUpper(a) == a, make([]string, 0)}
+		system[a] = c
+	}
+
+	c := system[a]
+	c.connected = append(c.connected, b)
+	system[a] = c
+
+	return system
+}
+
+func processInput(input *bufio.Scanner) map[string] cave {
+	system := make(map[string] cave)
+
+	for input.Scan() {
+		line := input.Text()
+
+		connection := strings.Split(line, "-")
+
+		system = connect(system, connection[0], connection[1])
+		system = connect(system, connection[1], connection[0])
+
+	}
+
+	return system
+}
+
+func router1(system map[string] cave, visited []string, current string) int {
+	if current == "end" {
+		return 1
+	}
+
+	sum := 0
+	outer:
+	for _, c := range system[current].connected {
+		for _, v := range visited {
+			if v == c {
+				if !system[c].large {
+					continue outer
+				}
+				break
+			}
+		}
+
+		var s int
+		s = router1(system, append(visited, c), c)
+		sum += s
+	}
+
+	return sum
+}
+
+func router2(system map[string] cave, visited []string, current string, twiced bool) int {
+	if current == "end" {
+		return 1
+	}
+
+	sum := 0
+	outer:
+	for _, c := range system[current].connected {
+		t := twiced
+		for _, v := range visited {
+			if v == c {
+				if !system[c].large {
+					if twiced || c == "start" {
+						continue outer
+					}
+					t = true
+				}
+				break
+			}
+		}
+
+		s := router2(system, append(visited, c), c, t)
+		sum += s
+	}
+
+	return sum
+}
+
+func main() {
+	challenge := aoc.New(2021, 12)
+
+	challenge.Test(`start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end`, []int{10, 36})
+
+	challenge.Test(`dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc`, []int{19, 103})
+
+	challenge.Test(`fs-end
+he-DX
+fs-he
+start-DX
+pj-DX
+end-zg
+zg-sl
+zg-pj
+pj-he
+RW-he
+fs-DX
+pj-RW
+zg-RW
+start-pj
+he-WI
+zg-he
+pj-fs
+start-RW`, []int{226, 3509})
+
+	challenge.Solution(1, func (input *bufio.Scanner) int {
+		return router1(processInput(input), []string{"start"}, "start")
+	})
+
+	challenge.Solution(2, func (input *bufio.Scanner) int {
+		return router2(processInput(input), []string{"start"}, "start", false)
+	})
+}