1. The Mishnah says that an Eruv on top of a very tall pole is valid.
2. The Mishnah implies that if the Eruv is on top of a reed that is attached to the ground, it is invalid.
3. Soft reeds are not considered like trees.
4. The Mishnah says that if a person put his Eruv in a cabinet and then lost the key, the Eruv is still valid.
5. The Gemara proceeds to give various explanations for why the Mishnah maintains that an inaccessible Eruv (#4) is a valid Eruv.
A BIT MORE
1. The top of the pole must be four Tefachim wide, in accordance with the rule that an Eruv is valid only if placed on an area that is four Tefachim wide.
2. This is not because the Eruv is inherently invalid, but rather the Rabanan invalidated it because of a concern that a person might uproot the reed in order to reach the Eruv.
3. Therefore, one is permitted to use them (e.g. to sit on them if a bunch are pressed together) on Shabbos.
4. Rebbi Eliezer argues that if the person does not know where the key is, the Eruv is invalid since he is essentially in a different place than his Eruv.
5. Rav and Shmuel explain that the case is where he can reach the Eruv by taking apart the cabinet in a permitted way. The Gemara further explains that this leniency applies only on Yom Tov (when the Rabanan relaxed their prohibitions for the purpose of essential Simchas Yom Tov) but not on Shabbos (and Rebbi Eliezer argues only with regard to Shabbos).