Meaning of Padmanabha
Symbolism of Vishnu:
Lord Vishnu is widely regarded as the Supreme Creator. In our religion he is shown reclining on the coils of a gigantic, seven-hooded serpent called Ananta-Sesha, in the middle of the cosmic milky ocean.
Both Vishnu and Ananta-Sesha and are eternally present through the periodic cosmic cycles of creation and destruction. The Vishnu Purana states that at the end of a “Day of Brahma” (1000 Mahā-Yugas = 1 Kalpa = 1 day (day only) of Brahma), when the entire creation has been dissolved, and the world has been converted into one vast ocean, “the Supreme reposes upon his mighty serpent couch amidst the deep: he awakes after a season, and again, as Brahma, becomes the author of creation.”
The above image is from Munneswaram Temple, Sri Lanka
The emerging lotus flower from the navel of Vishnu (vishnu nabhi), on which Brahma is seated is set to bring forth the material of creation. Hence, Vishnu is described as “lotus-navelled” (padma-nabha); and Brahma is termed the “navel-born” (nabhija).
About Padmanabha Swami Temple:
The above image is a 32 kg idol of Vishnu in Ananta Sayanam pose apart form the shiva linga which was shot of the treasures which were unearthed form temple.
As for the treasures of the temple it is a very old temple as many purana's like Brahma Purana, Matsya Purana, Varaha Purana, Skanda Purana etc, states about that shrine.
Late in the middle of 17th and 18th century the Anizham Thirunal Marthanda Varma took over the throne at the age of 22 from Ettuveetil Pillamar due to some conspiracy, and as the temple has 6 chambers labelled form 'A to F' has been present from the day of construction of temple, it was used to store the valuables of the ruling king as a safe deposit and only the king and the high priest of the temple were allowed to go inside the vault, until recently when chamber 'A' was opened.
NOTE: chamber B has not yet opened due to fear of bad omen as they did Ashtamangala Devaprasnam which might cause Divine displeasure
They also had gold coins of East India Company. It is assumed that when the British had war with the southern kings, they hid most of the treasures in that vault and was asked to seal it off and was never seen again until recently.