Vishnupad temple, Gaya, a sanctified place where the lord's pada (foot print) is worshiped

Vishnupad Temple, Gaya, entirely made of granite Myowncity.in

Vishnupad Mandir en.wikipedia.org

Vishnupada Mandir is an ancient temple in Gaya, Bihar  and the presiding deity is referred to as  Dharmasila. This Hindu temple, dedicated to Lord Vishnu, is entirely made of hard stone  and yet another feature is  it has the footprint (Padam)  of Lord Vishnu imprinted in the basaltic rock.. This place of worship is highly  sanctified because of the  visits of great Purushas - saints  such as  Ramanujacharya, Madhvacharya, Chaitanya Mahaprabhu and Sri Ramakrishna way back in the past. The peculiarity about this temple is there is no idol of Sri Vishnu in the sanctum. Here, due importance is given to Pada worship - puja (darshan of Lord's Pada). In Tamil Nadu, the tradition among the Hindus has been that when they  visit a Perumal temple, dedicated to Lord Vishnu either in a standing (Viswaroopam) posture or in Ananthasayanam (re-clining on the coiled bed of serpent Adishesa  posture), the temple priest (Bhattacharya) will normally ask the devotees to have Pada Dharsan.

Vishnu Pad, Gaya Sri Vishnu Pad temple, rvatemples.com
Vishnu pada at  Vushnu Pada Mandir, Gaya. Detechter
Above image; You can see the foot print of the Lord imprinted in the rock. The rim around is silver  plated. Puja is done to the Pada
which is 40 cm long. No idols in the sanctum. Nor does the lord have his consorts Sri Devi or Bhudevi as in the Vishnu temples of the south .................................. 
The legend has it that once a demon - Asura by the name of Gayasura, after doing penance for a long period of time, got a strange boon  from Lord Vishnu by which  whoever saw him would attain salvation (Moksham). So, no toil no sweat.   Soon, people, including the bad ones got the  salvation  from the demon without either long penance or doing good deeds like charity, etc.  Getting salvation, according to the Hindu Dharma, is not that easy. It requires long meditation on God, practice of celibacy, strict disciplined life and above all leading a  dharmic life, not deviating from the path of righteousness. 

Vishnupad Mandir in 1885,  en.wikipedia.org

To avoid  people with no moral uprightness and propriety  attaining salvation  from the Asura, Lord Vishnu asked Gayasura to go below the earth (pathal lok). Lord Vishnu   placed  his right foot on Asura  Gayasura's head and pushed him hard down below the earth. it is believed that Lord Vishnu's Pada -  foot print got pressed permanently on the  surface that we see even today. The footprint consists of nine different symbols including Shanku, Chakram and Gadham that are  said  to be weapons of the lord. When Gayasura, now pushed into earth, made a plea for food, Lord Vishnu gave him a boon that every day, someone would offer him food and whoever did so  would get salvation - free from rebirth. The Asura would come of his place in the nether world on the day he was not offered food. Everyday someone from some part of India comes there offers  Gayasura  food, according to the belief'. So, the  demon remains in his place - pathal lok quite contented. 

No information is available as to the builder of this temple. It seems, it has been around for a long time and is believed to have been visited by none other than Lord Rama and his consort Sita.  This historic temple was rebuilt in 1787. Thanks to the munificence of  Devi Ahilya Bai Holkar, the ruler of Indore. About 1 km southwest of the Vishnupad mandir is the Brahmajuni hill that can be accessed by a  flight of 1000 stone steps. From the hill top you can see the impressive Sri Vishnu temple.  With Vishnu's foot print in the center, the temple was built to a height of 30 meters facing East. The adjacent pavilion is supported by 8 rows of highly ornate pillars. The temple is made of grayish granitic rocks. The tower is in pyramidal shape and rises to a height of 100 feet. The tower has sloping sides with alternately indented and plain sections. The sections, in turn, are set at an angle to create a series of peaks joined at the top.

On the temple premises stands a big banyan tree Akshayabat where people conduct  the final rituals for the dead, in the belief, they will get salvation.    
https://en.wikipedia.org/wiki/Vishnupad_Mandir
https://www.rvatemples.com/listings/gaya-vishnupad-temple/