Why are tables created in hibernate deleted after stopping the server?

I run my site and it generates the tables in the database. I even can register in the database through the site and his select it shows that it has been registered. But if I stop the server, and see the tables in MySQL, the table is gone.

persistence looks like this:

<?xml version="1.0" encoding="UTF-8"?>

<persistence xmlns="http://java.sun.com/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
             version="2.0">
    <persistence-unit name="nome">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <class>br.com.nome.model.Tabela</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
            value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
            value="jdbc:mysql://localhost/bd" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="senha" />
            <property name="hibernate.dialect"
            value="org.hibernate.dialect.MySQL5InnoDBDialect" />
            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.format_sql" value="true" />
            <property name="hibernate.hbm2ddl.auto" value="update" />
        </properties>
    </persistence-unit>
</persistence>