The Umi-Sen Ken (海千拳 Umisenken?), translated variously as 1000 Seas Path', Sea of 1000 Fists', and Way of the Quiet/Silent Thief, was one of two secret and deadly styles of martial arts that Genma Saotome developed when he was younger, the other being the Yama-Sen Ken. However both styles were so destructive that they were later sealed away.
While both styles used the analogy of the human body as a house, they were originally intended for robbery and burglary. The Umi-Sen Ken's techniques specifically simulated the silent entry of a burglar through the back door and emphasized "stealth", utilising swift techniques and masking the user's presence. Because of this emphasis it was more difficult to master than the Yama-Sen Ken. Its weakness was that when the user became emotional or angry, they could be brought out of their emotionless state and their presence revealed to an opponent
Genma taught Ranma the Umi-Sen Ken when Ryu Kumon arrived in Furinkan and by chance learned that Nodoka was a Saotome, as he was searching for the Umi-Sen Ken scroll. By becoming emotionless, Ranma could hide the "presence" which warns other martial artists of danger, and using extreme speed could sneak up on and attack them. Ranma tried to take Ryu on twice, but was defeated each time. Following the second defeat he demanded that Genma teach him the Umi-Sen Ken. Genma took his son into the Tendo Dojo and shut the door to prevent Soun and Akane from observing. Telling Ranma that he would only teach him how the Umi-Senken worked once, Genma masked his presence and hit him with the Strike of the Spitting White Snake, knocking him out. Despite this, Ranma quickly ascertained how the Umi-Sen Ken functioned and began to train within the house, taking food, silverware and plates from the kitchen and putting them into Akane's drawers, stealing a section of newspaper that Soun was reading, and literally taking out the foundation of the living room while everyone else was having dinner. Although Akane saw him the first time he was in her room, Ranma improved and when he felt that he was ready, asked her to deliver a challenge letter and personal message to Ryu, who was staying at the Saotome home.
The two faced off at a local Buddhist temple in a match that would decide the fate of both styles. Ranma had an initial advantage until Ryu managed to upset him by threatening to reveal his secret to Nodoka, which prevented him from masking his presence. After he was forced to use the Cloth of Falling Stars and the follow up attacks, the fight escalated until both Ranma and Ryu used their respective ultimate attacks against each other. However the Deep-Sea Demon Wrap was able to counter the Ultimate Demon-God Multiple Strike, defeating Ryu. Although they both then sealed the techniques away, Nodoka still has the Umi-Sen Ken scroll, which she has been cutting up as material to make envelopes.
Though it can still be used quite effectively in general combat, the Umi-Sen Ken's primary purpose is to defeat a user of the Yama-Sen Ken, and so when two masters of each school fight, assuming their skill to be equal, it will be the Umi-Sen Ken that will always win.
- Opening move: the user takes a stance like that of a ghost, hands held limp in front of them with their arms slightly bent. The user then masks his presence or battle aura from an opponent, effectively rendering them invisible and able to strike at will provided that they can maintain their composure. They can still be seen by onlookers, however.
- Slashing Armored Fangs of Fury (鎧戸裂牙断 Gaito Retsuga Dan?) A counter to the Yama-Sen Ken's "Fanged Fingers of Steel Welcome-Gate" if it is aimed at the head by biting the attacker's hand.
- Stance of the Collapsing Brace (支柱落地勢 Shichū Rakuchi Sei?): Taking advantage of an attempt by the opponent to slam them to their ground after their hand is bitten, the user forces their legs open, unbalancing them briefly to set up for the next attack. Based on knocking loose floor braces without being caught by the collapsing floor.
- Strike of the Spitting White Snake (白蛇吐信掌 Haku Dato Shin Shō?): A rapid volley of strikes to the back, based on the concept of a burglar rapidly opening and closing drawers.
The following techniques form a three part sequence:
- Cloth of Falling Stars (護身流星布 Goshin Ryūsei Fu?): The user deploys a concealed furoshiki and swiftly ties it around their opponent's head. Ranma was forced to use this to break out of the "Crushing Embrace of the Killer Grip".
- Carp Body Flip (鯉魚翻身 Rigyo Honshin?): After using the Cloth of Falling Stars to trap the opponent's head, the user jumps out of their grip and flips their body like a carp to place themselves in position behind their opponent.
- Mountain Falling Into Sea (背山倒海態 Haizan Tōkai Tai?): Holding onto the furoshiki, the user pushes back with their legs against their opponent's thighs to force them down while trying to suffocate them. Ryu however was able to tear through the furoshiki, allowing him to breathe, and countered by wrapping the "Flight of the Tight-Bind Golden Threads" around Ranma's neck. The two were about to strangle each other until Soun cut the rope.
- Mega Cloth of Falling Stars (護身大流星布 Goshin Dai Ryūsei Fu?): The user deploys a large furoshiki and masks their presence in order to simultaneously stuff the furoshiki full of any nearby object, such as debris and their opponent's clothing, while tearing up the nearby ground with their feet. It is a set up for the final attack.
- Deep-Sea Demon Wrap (夜叉探海包 Yasha Tankai Hō?): The Umi-Sen Ken's ultimate attack, which specifically counters the Yama-Sen Ken's "Ultimate Demon-God Multiple Strike". After using the Mega Cloth of Falling Stars to prepare, the user gets an opponent's attention and jumps at them from above. As soon as the Ultimate Demon-God Multiple Strike is unleashed, the user drops the contents from the furoshiki above them, which "steals" the vacuum created by blocking it from above. This applies pressure and pulls the nearby air up, creating a reverse vacuum. This is so strong that that it sucks up the nearby loose earth to bury the opponent beneath a mound of dirt. The technique cannot be blocked or evaded.