Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #8

فإن قيل: قولكم أنه منذ ولد صاحب الزمان عليه السلام إلى وقتنا هذا مع طول المدة لا يعرف أحد مكانه ولا يعلم مستقره ولا يأتي بخبره من يوثق بقوله خارج عن العادة،لأن كل من اتفق له الاستتار عن ظالم لخوف منه على نفسه أو لغير ذلك من الأغراض يكون مدة استتاره قريبة ولا يبلغ عشرين سنة ولا يخفى أيضا على الكل في مدة استتارة مكانه ولا بد من أن يعرف فيه بعض أوليائه وأهل مكانه أو يخبر بلقائه وقولكم بخلاف ذلك.

If it is said: Your assertion that since the birth of the Master of the Age until this day, which is a long period of time, no one knows his location and residence, nor does anyone reliable brings any news about him. This is extraordinary, because everyone else who underwent hiding from an oppressor due to fear of his life or other reasons, he does not go into a very lengthy hiding and it does not take more than twenty years. Likewise, he does not hide from everyone the place of his hiding and some of his trusted followers and his family does know his location and brings news of his meeting. Your belief is very different from that.