diff --git a/2021/day12/Cargo.lock b/2021/day12/Cargo.lock
new file mode 100644
index 0000000..5e9bbe3
--- /dev/null
+++ b/2021/day12/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day12"
+version = "0.1.0"
diff --git a/2021/day12/Cargo.toml b/2021/day12/Cargo.toml
new file mode 100644
index 0000000..ba13763
--- /dev/null
+++ b/2021/day12/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "day12"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/2021/day12/README.md b/2021/day12/README.md
new file mode 100644
index 0000000..0b90f69
--- /dev/null
+++ b/2021/day12/README.md
@@ -0,0 +1,146 @@
+https://adventofcode.com/2021/day/12
+
+## \--- Day 12: Passage Pathing ---
+
+With your submarine's subterranean subsystems subsisting suboptimally, the
+only way you're getting out of this cave anytime soon is by finding a path
+yourself. Not just _a_ path - the only way to know if you've found the _best_
+path is to find _all_ of them.
+
+Fortunately, the sensors are still mostly working, and so you build a rough
+map of the remaining caves (your puzzle input). For example:
+
+[code]
+
+    start-A
+    start-b
+    A-c
+    A-b
+    b-d
+    A-end
+    b-end
+    
+[/code]
+
+This is a list of how all of the caves are connected. You start in the cave
+named `start`, and your destination is the cave named `end`. An entry like
+`b-d` means that cave `b` is connected to cave `d` \- that is, you can move
+between them.
+
+So, the above cave system looks roughly like this:
+
+[code]
+
+        start
+        /   \
+    c--A-----b--d
+        \   /
+         end
+    
+[/code]
+
+Your goal is to find the number of distinct _paths_ that start at `start`, end
+at `end`, and don't visit small caves more than once. There are two types of
+caves: _big_ caves (written in uppercase, like `A`) and _small_ caves (written
+in lowercase, like `b`). It would be a waste of time to visit any small cave
+more than once, but big caves are large enough that it might be worth visiting
+them multiple times. So, all paths you find should _visit small caves at most
+once_ , and can _visit big caves any number of times_.
+
+Given these rules, there are `_10_` paths through this example cave system:
+
+[code]
+
+    start,A,b,A,c,A,end
+    start,A,b,A,end
+    start,A,b,end
+    start,A,c,A,b,A,end
+    start,A,c,A,b,end
+    start,A,c,A,end
+    start,A,end
+    start,b,A,c,A,end
+    start,b,A,end
+    start,b,end
+    
+[/code]
+
+(Each line in the above list corresponds to a single path; the caves visited
+by that path are listed in the order they are visited and separated by
+commas.)
+
+Note that in this cave system, cave `d` is never visited by any path: to do
+so, cave `b` would need to be visited twice (once on the way to cave `d` and a
+second time when returning from cave `d`), and since cave `b` is small, this
+is not allowed.
+
+Here is a slightly larger example:
+
+[code]
+
+    dc-end
+    HN-start
+    start-kj
+    dc-start
+    dc-HN
+    LN-dc
+    HN-end
+    kj-sa
+    kj-HN
+    kj-dc
+    
+[/code]
+
+The `19` paths through it are as follows:
+
+[code]
+
+    start,HN,dc,HN,end
+    start,HN,dc,HN,kj,HN,end
+    start,HN,dc,end
+    start,HN,dc,kj,HN,end
+    start,HN,end
+    start,HN,kj,HN,dc,HN,end
+    start,HN,kj,HN,dc,end
+    start,HN,kj,HN,end
+    start,HN,kj,dc,HN,end
+    start,HN,kj,dc,end
+    start,dc,HN,end
+    start,dc,HN,kj,HN,end
+    start,dc,end
+    start,dc,kj,HN,end
+    start,kj,HN,dc,HN,end
+    start,kj,HN,dc,end
+    start,kj,HN,end
+    start,kj,dc,HN,end
+    start,kj,dc,end
+    
+[/code]
+
+Finally, this even larger example has `226` paths through it:
+
+[code]
+
+    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
+    
+[/code]
+
+_How many paths through this cave system are there that visit small caves at
+most once?_
+
diff --git a/2021/day12/input/example0 b/2021/day12/input/example0
new file mode 100644
index 0000000..6fd8c41
--- /dev/null
+++ b/2021/day12/input/example0
@@ -0,0 +1,7 @@
+start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end
diff --git a/2021/day12/input/example1 b/2021/day12/input/example1
new file mode 100644
index 0000000..7048cd5
--- /dev/null
+++ b/2021/day12/input/example1
@@ -0,0 +1,5 @@
+    start
+    /   \
+c--A-----b--d
+    \   /
+     end
diff --git a/2021/day12/input/example2 b/2021/day12/input/example2
new file mode 100644
index 0000000..ef84176
--- /dev/null
+++ b/2021/day12/input/example2
@@ -0,0 +1,10 @@
+start,A,b,A,c,A,end
+start,A,b,A,end
+start,A,b,end
+start,A,c,A,b,A,end
+start,A,c,A,b,end
+start,A,c,A,end
+start,A,end
+start,b,A,c,A,end
+start,b,A,end
+start,b,end
diff --git a/2021/day12/input/example3 b/2021/day12/input/example3
new file mode 100644
index 0000000..62cc714
--- /dev/null
+++ b/2021/day12/input/example3
@@ -0,0 +1,10 @@
+dc-end
+HN-start
+start-kj
+dc-start
+dc-HN
+LN-dc
+HN-end
+kj-sa
+kj-HN
+kj-dc
diff --git a/2021/day12/input/example4 b/2021/day12/input/example4
new file mode 100644
index 0000000..bbafd51
--- /dev/null
+++ b/2021/day12/input/example4
@@ -0,0 +1,19 @@
+start,HN,dc,HN,end
+start,HN,dc,HN,kj,HN,end
+start,HN,dc,end
+start,HN,dc,kj,HN,end
+start,HN,end
+start,HN,kj,HN,dc,HN,end
+start,HN,kj,HN,dc,end
+start,HN,kj,HN,end
+start,HN,kj,dc,HN,end
+start,HN,kj,dc,end
+start,dc,HN,end
+start,dc,HN,kj,HN,end
+start,dc,end
+start,dc,kj,HN,end
+start,kj,HN,dc,HN,end
+start,kj,HN,dc,end
+start,kj,HN,end
+start,kj,dc,HN,end
+start,kj,dc,end
diff --git a/2021/day12/input/example5 b/2021/day12/input/example5
new file mode 100644
index 0000000..65f3833
--- /dev/null
+++ b/2021/day12/input/example5
@@ -0,0 +1,18 @@
+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
diff --git a/2021/day12/input/input b/2021/day12/input/input
new file mode 100644
index 0000000..00a5183
--- /dev/null
+++ b/2021/day12/input/input
@@ -0,0 +1,25 @@
+fw-ll
+end-dy
+tx-fw
+tx-tr
+dy-jb
+ZD-dy
+dy-BL
+dy-tr
+dy-KX
+KX-start
+KX-tx
+fw-ZD
+tr-end
+fw-jb
+fw-yi
+ZD-nr
+start-fw
+tx-ll
+ll-jb
+yi-jb
+yi-ll
+yi-start
+ZD-end
+ZD-jb
+tx-ZD
diff --git a/2021/day12/src/main.rs b/2021/day12/src/main.rs
new file mode 100644
index 0000000..5aa1eda
--- /dev/null
+++ b/2021/day12/src/main.rs
@@ -0,0 +1,144 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
+use std::io::stdin;
+use std::io::BufRead;
+
+fn main() {
+    println!("Hello, world!");
+    let arg = std::env::args().nth(1).unwrap_or_else(String::new);
+    match arg.as_str() {
+        "part1" => part1(),
+        _ => part2(),
+    }
+}
+
+fn part1() {
+    println!("Part1");
+
+    let mut map: HashMap<String, Vec<String>> = HashMap::new();
+
+    for line in stdin().lock().lines() {
+        let line = line.unwrap();
+        println!("{}", line);
+        let mut parts = line.split('-');
+        let cave_a = parts.next().unwrap().to_string();
+        let cave_b = parts.next().unwrap().to_string();
+
+        let cave_a_connections = map.entry(cave_a.clone()).or_insert_with(Vec::new);
+        cave_a_connections.push(cave_b.clone());
+
+        let cave_b_connections = map.entry(cave_b.clone()).or_insert_with(Vec::new);
+        cave_b_connections.push(cave_a);
+    }
+
+    println!("{:?}", map);
+
+    let mut incomplete_paths = Vec::new();
+    let mut paths = Vec::new();
+
+    incomplete_paths.push(vec!["start".to_string()]);
+
+    while !incomplete_paths.is_empty() {
+        let path = incomplete_paths.pop().unwrap();
+        let last_cave = path.last().unwrap();
+
+        let options = map.get(last_cave).unwrap();
+
+        for option in options {
+            let first_char = option.chars().next().unwrap();
+            if first_char.is_lowercase() {
+                let has_been_visited = path.iter().any(|cave| cave == option);
+                if has_been_visited {
+                    continue;
+                }
+            }
+            let mut new_path = path.clone();
+            new_path.push(option.clone());
+
+            if option == "end" {
+                paths.push(new_path);
+            } else {
+                incomplete_paths.push(new_path);
+            }
+        }
+    }
+
+    for path in paths.iter() {
+        println!("{:?}", path);
+    }
+
+    let path_count = paths.len();
+    dbg!(path_count);
+}
+
+fn part2() {
+    println!("Part2");
+
+    let mut map: HashMap<String, Vec<String>> = HashMap::new();
+
+    for line in stdin().lock().lines() {
+        let line = line.unwrap();
+        println!("{}", line);
+        let mut parts = line.split('-');
+        let cave_a = parts.next().unwrap().to_string();
+        let cave_b = parts.next().unwrap().to_string();
+
+        let cave_a_connections = map.entry(cave_a.clone()).or_insert_with(Vec::new);
+        cave_a_connections.push(cave_b.clone());
+
+        let cave_b_connections = map.entry(cave_b.clone()).or_insert_with(Vec::new);
+        cave_b_connections.push(cave_a);
+    }
+
+    println!("{:?}", map);
+
+    let mut incomplete_paths = Vec::new();
+    let mut paths = Vec::new();
+
+    incomplete_paths.push(vec!["start".to_string()]);
+
+    while !incomplete_paths.is_empty() {
+        let path = incomplete_paths.pop().unwrap();
+        let last_cave = path.last().unwrap();
+
+        let options = map.get(last_cave).unwrap();
+
+        for option in options {
+            let first_char = option.chars().next().unwrap();
+
+            if option == "start" {
+                continue;
+            }
+            if first_char.is_lowercase() {
+                let has_been_visited = path.iter().any(|cave| cave == option);
+                let mut visited_small_caves = path
+                    .iter()
+                    .filter(|cave| cave.chars().next().unwrap().is_lowercase())
+                    .collect::<Vec<&String>>();
+                let count_small_caves = visited_small_caves.len();
+                visited_small_caves.sort();
+                visited_small_caves.dedup();
+                let count_unique_small_caves = visited_small_caves.len();
+                let is_any_small_cave_visited_twice = count_small_caves != count_unique_small_caves;
+                if is_any_small_cave_visited_twice && has_been_visited {
+                    continue;
+                }
+            }
+            let mut new_path = path.clone();
+            new_path.push(option.clone());
+
+            if option == "end" {
+                paths.push(new_path);
+            } else {
+                incomplete_paths.push(new_path);
+            }
+        }
+    }
+
+    for path in paths.iter() {
+        println!("{:?}", path);
+    }
+
+    let path_count = paths.len();
+    dbg!(path_count);
+}